UC-NRLF 


•WVANCED  SERIES 


•"•••*•« 

5MY 


M.D.,  Edinburgh  University. 

19.  METALLURGY.     By  John  Mayer,  F.C.S.,  Glasgow.- 

20.  NAVIGATION.     By  Henry  Evers,  LL.D.,  Plymouth. 

21.  NAUTICAL  ASTRONOMY.     By  Henry  Evers,  LL.D. 

22A  STEAM  AND  THE  STEAM  ENGINE— LAND  AND   MARH 

By  Henry  Evers,  LL.D.,  Plymouth. 
221)  STEAM    AND    STEAM    ENGINE— LOCOMOTIVE.    By    Her 

Evers,  LL.D.,  Plymouth. 

23.  PHYSICAL  GEOGRAPHY.     By  John  Macturk,  F.R.G.S. 

24.  PRACTICAL  CHEMISTRY.     By  John  Howard,  London. 

25.  ASTRONOMY.     By  J.  J.  Plummer,  Observatory,  Durham. 


A 


Adapted  to  tht 


unifa 


PRACTIC 

F.  A.  B 
MACHIN 

Tomkin 
BUILDI 
NAVAL 

ByS. 
PURE 

Leicest 
THEORE 

of  Natu: 
APPLIED 

College, 
ACOUSTI 

Derby. 
MAGNET 

Ph.D., 
INORGA 

Profess* 

2  Vols. 
ORGANIC 

Lecture 


,and 


cloth 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


E. 


OFF. 


rod.,) 


L.D., 
B.A., 

S.E., 

gow 

C.S., 


GEOLOGY.     By  John  Young,  M.D.,  Professor  of  Natural  History, 

Glasgow  University. 
ANIMAL  PHYSIOLOGY.     By  J.  Cleland,  M.D.,  F.R.S.,  Professor 

of  Anatomy  and  Physiology,  Gal  way. 

ZOOLOGY.     By  E.  Ray  Lankester,  M.A.,  (Oxon.,)  London. 
VEGETABLE   ANATOMY  AND    PHYSIOLOGY.    By  J.    H. 

Balfour,  M.D.,  Edinburgh  University. 
SYSTEMATIC  AND  ECONOMIC  BOTANY.     By  J.  H.  Balfour, 

M.D.,  Edinburgh  University. 

METALLURGY.     By  W.  H.  Greenwood,  A.R.S.M.     2  Vols. 
NAVIGATION.     By  Henry  Evers,   LL.D.,  Professor  of  Applied 

Mechanics,  Plymouth. 

NAUTICAL  ASTRONOMY.     By  Henry  Evers,  LL.D.,  Plymouth. 
STEAM  AND  THE  STEAM  ENGINE— LAND,  MARINE,  AND 

LOCOMOTIVJE.     By  Henry  Evers,  LL.D.,  Plymouth. 
PHYSICAL  GEOGRAPHY.     By  John  Young,  M.D.,  Professor  of 

Natural  History,  Glasgow  University. 


(jlutem's 


ANIMAL   PHYSIOLOGY: 


THE 


STKUCTUEE  AND  FUNCTIONS 

OF   THE 

HUMAN    BODY. 


BY 


JOHN  JCLEL AND, /M.D.,    F.E.S., 

tJRUFESSOB  OF  ANATOMY  AND  PHYSIOLOGY  IN  QUEEN'S  COLLEGE,  GALWAT^/ 


'WITH    158    ENGRAVINGS.^ 


NEW  YOUK: 
G.  P.  PUTNAM'S  SONS, 

FOUETH  AVENUE  AND  TWENTY-THIRD  STREET. 


PREFACE. 


THE  principal  object  of  this  book  is  to  supply  to  readers  pre- 
viously unacquainted  with  anatomical  details  as  complete  an 
account  as  possible  of  the  functions  of  the  body.  In  doing 
this,  the  author  has  kept  constantly  in  view  the  desire  of 
the  Publishers  to  supply  the  information  required  for  the 
Advanced  Course  of  the  Directory  of  the  Science  and  Art 
Department;  and  at  the  same  time  has  sought  to  furnish 
to  the  junior  student  of  medicine  a  compendium  of  physi- 
ology which  may  assist  him  in  obtaining  a  clear  idea  of  the 
principles  of  the  science,  and  prepare  him  for  the  perusal  of 
works  of  more  elaborate  character. 

Necessarily  such  a  book  is,  to  some  extent,  a  compilation; 
but  it  is  hoped  that,  in  grouping  of  facts,  and  in  directing 
the  reader's  mind  to  just  conceptions  and  conclusions,  this 
manual  may  be  found  to  be  something  more  than  a  mere 
collection  of  details. 

With  a  few  exceptions,  the  Illustrations  have  been  engraved 
from  pencil  sketches,  the  majority  of  them  original,  and  others 
taken  from  sources  which  are  acknowledged;  and  the  author 
takes  this  opportunity  of  thanking  the  Engraver,  Mr.  STEPHEN 
MILLER,  of  Glasgow,  for  the  care  which  he  has  bestowed  on 
them, 

J.  C. 
GALWAY,  Oct.,  1873. 


.     J 


CONTENTS, 


CHAPTER  I. 

INTRODUCTION,  FUNCTIONS  OF  ANIMALS,  NUCLEATED  CORPUSCLES 

PAGE 

Physiology  —  Anatomy — Biology —  Histology  —  Organisms — 
Organic  Matter — Animals  and  Vegetables — The  Functions 
of  Animals — Nutrition — Reproduction — Sensation — Move- 
ment— Albuminoids — Protoplasm — Amoeba  —  Nucleated 
Corpuscles,  ------  .9 

CHAPTER  II. 

THE  CONNECTIVE  TISSUES. 

Definition — Areolar    Tissue — Connective-Tissue-Corpuscles — 

White  Fibrous  Tissue — Elastic  Tissue — Adipose  Tissue,  -        20 

CHAPTER  III, 

THE  SKELETON. 

Definition — Vertebral  or  Spinal  Column — Costal  Cartilages-— 
Breast  Bone  or  Sternum — Collar  Bone  or  Clavicle — 
Scapula  or  Shoulder-blade — Humerus — Radius  and  Ulna 
— Carpal  and  Metacarpal  Bones — Phalanges — Sacrum 
— Coccyx — Pelvis — Thigh  Bone  or  Femur — Tibia  and 
Fibula— Bones  of  Foot— Atlas  and  Axis— The  Skull— 
Bones  of  the  Face — Peculiarities  of  Human  Skeleton — 
Skeletal  Textures  —  Cartilage  —  Bone  —  The  Joints  — 
Mechanics  of  the  Skeleton — Centre  of  Gravity — Move- 
ments of  the  Skeleton — Levers,  -  -  -  27 

CHAPTER  IV. 

MUSCLES. 

Muscular  Fibre — Striped  Muscular  Tissue — Unstriped  Mus- 
cular Tissue — Composition  of  Muscle — Muscular  Con- 
traction— Irritability — Rigor  Mortis,  -  •  53 


6  CONTENTS. 

CHAPTER  V, 

FREE  SURFACES,  EPITHELIUM,  SECRETION,  INTEGUMENT. 

PAGE 

Description  of  Free  Surfaces — Epithelium — Varieties  of  Epi- 
thelium— Secretion — Integument — Sensibility  of  the  Skin 
— Glands  of  the  Skin — Perspiration — Epidermal  Append- 
ages, .....  60 

CHAPTER  VL 

ALIMENTATION. 
The  Waste  and  Repair  of  the  Body— Different  Kinds  of  Food,       76 

CHAPTER  VII. 

DIGESTION. 

Processes  of  Digestion — The  Teeth — Development — Course  of 
the  Ingesta  —  Mastication  —  Deglutition  —  (Esophagus — 
Stomach — Pylorus — Duodenum — Jejunum — Ileum — Ileo- 
colic  Valve — Great  Intestine — Digestive  Tube  in  Different 
Animals — Digestive  Fluids — Saliva — Gastric  Juice — Gas- 
tric Follicles — Mucous  Membrane  of  Small  Intestine — 
Pancreatic  Juice — Bile — Contents  of  Great  Intestine — 
Closed  Follicles,  ....  -  82 

CHAPTER  VIII. 

THE  BLOOD. 

Amount  of  Blood  in  the  Body — Rough  Analysis  of  Blood— 
Red  and  White  Blood  Corpuscles — Chemical  Composition 
of  Blood — Coagulation — Gaseous  Contents — Arterial  and 
Venous  Blood,  ...  .  105 

CHAPTER  IX, 
CIRCULATION. 

Circulation  in  Different  Animals — Heart  and  Great  Vessels  of 
Fish — Heart  and  Great  Vessels  of  Frog — Human  -Heart 
and  Vessels  —  Action  of  the  Heart  —  Arterial  Valves  — 
Auriculo- ventricular  Valves  —  Sounds  —  Impulse  — Fre- 
quency of  Pulsation — Nervous  Control  of  Heart — The 
Arteries — Pulsation  in  the  Arteries — The  Capillaries — 
The  Veins — Venous  Valves — Velocity  of  Blood  through 
the  System — Portal  System,  -  -  -  114 


CONTENTS.  % 

CHAPTER  X. 
RESPIRATION  AND  TEMPERATURE. 

PAG3 

Object  of  Respiration — Various  Kinds  of  Respiratory  Organs — 
Lungs  of  Different  Animals — Human  Windpipe  and  Lungs 
— Infundibula — Air  Cells — Means  by  which  Air  is  intro- 
duced into  the  Lungs — Expansion  and  Diminution  of  the 
Chest — The  Diaphragm — Movements  of  the  Ribs — Rate 
of  Respiration — Vital  Capacity — The  Atmosphere — The 
Air  Exhaled — The  Oxygen  Inhaled — Obstruction  of  Re- 
spiration — Vitiated  Air — Disinfectants — Ventilation — 
Internal  Temperature,  -  -  133 

CHAPTER  XI. 
ABSORPTION. 

Means  by  which  the  Blood  is  Replenished — Absorbent  System 
— The  Lymphatics — Lymphatic  Glands — Lacteals — Chyle 
— Chyme — Villi — Passage  of  Liquids  by  Osmosis,  -  150 

CHAPTER  XII. 

THE  DUCTLESS  GLANDS,  THE  LIVER,  AND  THE  KIDNEYS. 

The  Ductless  Glands— The  Thyroid  Body— The  Thymus  Gland 
— The  Suprarenal  Capsules — The  Spleen — The  Liver — 
The  Bile — Glycogen — Uses  of  the  Liver — The  Kidneys — 
The  Urinary  Bladder — The  Urine — The  Formation  of 
Urea,  -  -  159 

CHAPTER  XIII. 

THE  NERVOUS  SYSTEM. 

Elements  of  the  Nervous  System — Nervous  Action — Functions 
of  the  Nervous  System — Properties  of  Living  Nerve — 
Nervous  Tissues  —  Nerve  -  Fibres  —  Nerve  -  Corpuscles  — 
Consistence  of  Nervous  Tissues — Cerebro-Spinal  Axis — 
The  Spinal  Cord — The  Spinal  Nerves — Functions  of  the 
Roots  of  the  Spinal  Nerves — Physiology  of  the  Spinal 
Cord,  -  -  176 

CHAPTER  XIV. 

THE  NERVOUS  SYSTEM — Continued. 

Structure  of  the  Encephalon — The  Cerebellum — The  Medulla 
Oblongata  —  Cerebrum  —  The  Cerebral  Hemispheres  — 
The  Brain  in  Different  Animals — Development  of  the 


CONTENTS* 


Brain — Cranial  Nerves — Functions  of  the  Different  Parts 
of  the  Encephalon — Connection  of  the  Hemispheres  with 
the  Faculties  of  the  Mind  — Mental  Operations  required 
in  Talking — Sleep — Dreams — Apparitions  —  Somnambul- 
ism— The  Sympathetic  System — Nervous  Supply  of  the 
Heart  and  Blood-vessels,  -  -  -  -193 

CHAPTER  XV. 

THE  SENSES. 

Common  Sensation — Localization  of  Impressions — Pain — Smell 
— Nasal  Fossse — The  Olfactory  Nerves — Olfactory  Cells — 
Taste — The  Tongue — Taste-Cones — Varieties  of  Tastes — 
Organ  of  Jacobson — Vision — The  Eyeball — Sclerotic — 
Choroid  Coat — Retina — Lens  and  Humours — Develop- 
ment of  the  Eye — Simple  Forms  of  Eyes — Sensibility  of 
Retina — Spherical  and  Chromatic  Aberration — Accommo- 
dation to  Distances — Muscles  of  the  Eyeball — Double 
Vision — Inversion — Distance — Effects  of  Distance  on  the 
Eyes — Appearance  of  Solidity — Duration  of  Impressions 
— Colour-blindness — Ocular  Spectra — Phosphenes — Lach- 
rymal Apparatus — Lachrymal  Gland — Hearing — The  Ear 
• — Cartilage  and  Muscles  of  External  Ear — The  Tym- 
panum— The  Tympanic  Ossicles — Membranous  Labyrinth 
—The  Cochlea— Mode  of  Action  of  Internal  Ear,  -  218 

CHAPTER  XVI. 

VOICE  AND  SPEECH. 

Voice — Cartilages  of  the  Larynx — Vocal  Cords — Muscles  of  the 

Larynx — Pitch  of  Voice — Speech — Mechanism  of  Speech,       '267 

CHAPTER  XVII. 

REPRODUCTION  AND  DEVELOPMENT. 

Modes  of  Multiplication — Sex — Reproduction  of  Lost  Parts 
—The  Ovum— The  Uterus— The  Ovaries— The  Testes 
• — Spermatozoa — The  Embryo — The  Primitive  Groove — 
The  Chorda  Dorsalis— The  Dorsal  and  Ventral  Plates— 
The  Cephalic  Plate — Development  of  the  Vascular. Sys- 
tem—  Closure  of  the  Amnion — Allantois  —  Placenta — • 
Footal  Circulation — Period  of  Gestation — Milk — Growth 
after  Birth— Death,  '  •  273 

GLOSSARY,  ....      303 

INDEX,     -  .  .  .  .  315 


ANIMAL  PHYSIOLOGY. 


'CHAPTEE  I. 

INTRODUCTION— FUNCTIONS  OF  ANIMALS— NUCLEATED 
CORPUSCLES. 

1.  THE  world  around  us  is  divisible  into  the  organic  and 
inorganic  worlds ;  the  organic  world  including  all  bodies 
which  either  are  or  have  been  alive,  and  the  inorganic  com- 
prising all  others. 

Physiology  is  the  study  of  the  healthy  operations  which 
take  place  in  living  beings;  and  when  the  word  is  used 
without  qualification,  it  is  customary  to  consider  that  special 
reference  to  the  physiology  of  the  human  body  is  intended  : 
still,  in  its  widest  signification,  it  refers  to  all  living  beings, 
both  animal  and  vegetable. 

It  is  a  science  which  goes  hand  in  hand  with  Anatomy, 
the  study  of  the  structure  of  living  beings;  for,  as  is  the 
case  with  an  artificial  mechanism,  so  also  with  the  body,  an 
acquaintance  with  its  structure  is  required  to  explain  the 
way  in  which  it  works. 

Anatomy  and  physiology  are  not,  however,  co-extensive. 
On  the  one  hand,  there  is  much  physiology  which  has  little 
apparent  connection  with  anatomy;  and,  on  the  other,  in  the 
present  state  of  science,  there  is  much  anato'my  which  can 
be  studied  without  special  reference  to  physiology.  In  fact, 
when  the  anatomist  rises  above  the  mere  description  of  the 
particular  objects  before  him,  he  examines  structures  from 
two  points  of  view,  one  of  which  is  the  physiological,  and 
has  regard  to  their  fitness  to  servo  some  purpose  useful  to 
the  being  to  which  they  belong,  while  the  other  is  called  the 
morphological  view,  and  looks  to  the  structural  affinities  of 


10  ANIMAL  PHYSIOLOGY. 

I 

parts  in  tlie  same  or  in  different  species;  for  example,  the 
relations  of  the  human  limbs  one  to  the  other  and  to  those 
of  other  animals. 

There  is  one  department  of  observation  in  which  the 
studies  of  the  anatomist  and  physiologist  become  identical, 
namely,  Development  5  in  it  series  of  forms  are  met  with, 
important  as  such  to  the  anatomist,  even  in  a  strictly  mor- 
phological respect,  while  by  the  physiologist  they  are  viewed 
as  phenomena  of  action  of  the  most  remarkable  kind,  peculiar 
to  living  beings. 

When  physiological  investigation  diverges  from  anatomy, 
it  comes  into  close  connection  with  other  branches  of  science. 
For  not  only  have  living  bodies  a  structure,  but  they  consist 
of  components  subject  to  the  laws  which  govern  matter  in 
the  inorganic  world.  Thus  the  body  consists  of  chemical  con- 
stituents, and  many  of  the  processes  taking  place  within  it 
are  of  a  chemical  nature.  Its  materials  are  also  subject 
to  the  ordinary  laws  of  physics  :  scattered  through  it  are 
varieties  of  mechanical  appliances ;  special  parts  are  set  aside 
for  optical  and  acoustic  purposes;  and  others  exhibit  electrical 
phenomena  of  a  very  remarkable  description.  The  study  of 
physiology  is  therefore  very  dependent  on  both  chemistry 
and  physics.  Its  connection,  however,  with  these  subjects  is 
of  a  different  nature  from  its  connection  with  anatomy;  for 
anatomy  and  physiology  are  two  closely  associated  depart- 
ments of  Biology,  or  the  science  of  life;  while  the  bond  which 
loins  biology  to  chemistry  and  physics  is  simply  this,  that 
living  bodies,  being  composed  of  matter,  are  subject  to  the 
laws  of  matter,  besides  exhibiting  additional  laws  peculiar 
to  themselves  and  termed  vital. 

In  the  following  pages  attention  will  be  principally  directed 
to  human  physiology,  but  occasional  reference  will  be  made 
to  peculiarities  of  function  in  other  animals;  and  while 
matters  which  are  peculiar  to  man  will  be  pointed  out,  it 
will  become  apparent  that  all  the  larger  facts  of  function,  as 
well  as  structure,  are  common  to  man  and  other  animals. 
Indeed,  our  knowledge  of  human  physiology  is  largely 
dependent  on  experiments  on  dogs,  rabbits,  horses,  birds,  and 
even  frogs.  It  will  also  be  our  business  to  enter  into  various 
anatomical  details,  to  give  the  student  a  knowledge  of  the 


ORGANIC  MATTEH.  11 

structures  principally  implicated  in  the  physiological  pro- 
cesses to  be  explained;  and,  in  particular,  it  will  be  necessary 
to  describe  the  textures  or  tissues  of  the  different  parts,  which 
in  great  measure  require  the  aid  of  the  microscope  for  their 
examination.  This  is  the  department  of  anatomy  termed 
Histology. 

2.  Living  bodies  are  termed  Organisms,  because  they  are 
composed  of  organs,  or  parts  devoted  to  different  purposes ; 
and  the  purpose  to  which  any  organ  is  devoted  is  called  its 
function. 

Organs  are  of  various  degrees  of  complexity.  In  organisms 
of  the  higher  or  more  complicated  description,  bodies  com- 
parable with  organisms  of  a  very  simple  or  rudimentary  kind 
exist  as  textural  elements.  Such  textural  elements  enter  into 
the  formation  of  more  complex  textural  organs  (e.g.,  arteries 
in  animals,  and  vascular  bundles  in  plants),  which  are  dis- 
tributed as  component  parts  of  a  variety  of  special  organs, 
such  as  eye,  ear,  liver,  brain,  etc.,  often  the  only  parts  alluded 
to  when  the  term  special  organ  is  used. 

3.  All  organisms  are  in  great  part  composed,  particularly 
their  more  active  portions,  of  chemical  combinations  of  a 
complex  kind,  called  organic  matter,  together  with  which 
there  are  always,  in  addition,  various  mineral  constituents 
and  water  entering  into  their  composition.     The  most  dis- 
tinctive character  of  organic  matter  is  that  it  is  combustible, 
becoming  black  when  heated  over   a  flame  ;    and,   as  this 
blackness  is  due  to  carbon,  it  disappears  on  further  exposure 
to  heat  and  air,  leaving  the  ash  or  non-volatile   mineral 
constituents  which  are  always  associated  with  organic  matter. 

Organic  matter  is  divisible  into  two  groups  of  substances, 
which  are  distinguished  as  nitrogenous,  and  non-nitrogenous 
or  carbonaceous;  the  first  containing  carbon,  hydrogen, 
oxygen,  and  nitrogen,  and  the  second  having  no  nitrogen  in 
their  constitution.  The  products  of  the  complete  combustion 
of  carbonaceous  matters  are  carbonic  acid  and  water,  while 
nitrogenous  substances  yield  ammonia  in  addition. 

The  attraction  of  both  carbon  and  hydrogen  for  oxygen  is 
very  great.  Carbonic  acid,  consisting  of  one  equivalent  or 
combining  proportion  of  carbon  and  two  of  oxygen,  is  the 
compound  which  is  formed  when  carbon  is  freely  exposed  to 


12  ANIMAL    PHYSIOLOGY. 

oxygen  at  a  high  temperature;  and,  when  oxygen  and 
hydrogen  gases  are  mixed,  and  a  light  applied  to  them,  they 
combine  with  explosion,  producing  water,  which  consists  of 
two  equivalents  of  hydrogen  and  one  of  oxygen.  Ammonia 
consists  of  one  of  nitrogen  and  three  of  hydrogen;  and,  in 
the  complete  combustion  of  organic  matter,  this  hydrogen 
may  be  obtained  partly  from  the  organic  matter  itself,  and 
partly  from  water,  the  oxygen  of  which  is  used  in  the  forma- 
tion of  carbonic  acid.  In  less  perfect  combustion,  cyanogen 
in  a  state  of  combination  may  be  evolved  instead  of  ammonia, 
by  the  nitrogen  of  the  organic  matter  combining  with  part 
of  the  carbon,  in  the  proportion  of  one  equivalent  of  carbon 
to  one  of  nitrogen. 

The  combustibility  of  organic  matter  depends  on  its  con- 
tained oxygen  being  less  than  sufficient  to  combine  with  its 
carbon  and  hydrogen  to  form  carbonic  acid  and  water,  and 
on  the  complexity  of  its  molecules.  While  substances  found 
native  in  the  inorganic  world  consist  of  elements  grouped  in 
pairs,  in  which  the  number  of  equivalents  of  the  one  sub- 
stance bears  a  simple  proportion  to  the  equivalents  of  the 
other,  organic  substances  present  groups  of  three,  four,  or 
more  elements  gathered  together  in  common  union,  with 
many  equivalents  of  each  combined  in  one  molecule,  often 
in  proportions  by  no  means  simple;  and  more  especially  are 
the  molecules  of  the  nitrogenous  constituents  of  the  textures 
complex. 

The  oxidation  of  organic  matters  may  take  place  Ly  other 
means  besides  a  burning  heat.  Thus  it  occurs  in  the  form 
of  putrefaction  at  much  lower  temperatures,  especially  when 
aided  by  abundant  moisture.  So  also,  oxidation  of  organic 
matter  and  the  resolution  thereof  either  into  carbonic  acid, 
water,  and  ammonia,  or  into  products  of  less  complete 
decomposition,  take  place  in  the  interior  of  organisms  during 
life,  and  are  sometimes  alluded  to  under  the  name  of  com- 
bustion. 

4.  The  organic  world  is  divisible  into  two  kingdoms,  the 
animal  and  the  vegetable.  The  power  of  building  up  the 
complex  molecules  of  organic  matter  from  the  separate 
elements,  or  such  simple  combinations  as  carbonic  acid, 
water,  and  ammonia,  is  peculiar  to  vegetables,  while  intelli- 


FUNCTIONS   OF   ANIMALS.  13 

gence  is  confined  to  animals.  All  vegetables,  however,  do 
not  possess  the  building  power ;  it  is  apparently  a  property 
belonging  exclusively  to  green  parts :  and  all  animals  do  not 
possess  intelligence;  but,  on  the  contrary,  it  may  be  assumed 
to  be  entirely  absent  from  the  lowest  forms,  while  it  appears 
in  obscure  and  gradual  dawnings  in  others. 

In  the  region  of  the  minute  and  simple  beginnings  of  life, 
the  animal  and  vegetable  kingdoms  touch  one  another,  and 
there  may  even  be  a  common  territory  including  beings 
which  have  no  claim  to  be  classed  in  one  rather  than  the 
other.  But,  growth  and  reproduction  being  the  highest 
functions  of  vegetable  life,  while  intelligence  is  the  highest 
aim  exhibited  in  the  animal  series,  vegetable  and  animal 
forms  rapidly  diverge  as  they  become  complex,  so  that  those 
of  a  highly  developed  description  in  the  one  kingdom  cease 
to  have  any  resemblance  to  those  of  the  other. 

5.  The  functions  of  animals  may  be  enumerated  as  nutri- 
tion, reproduction,  sensory  functions,  and  movement.  The  first 
two  of  these,  being  equally  characteristic  of  animals  and 
vegetables,  are  sometimes  termed  functions  of  organic  life ; 
while  those  varieties  of  the  other  two  which  constitute  sensa- 
tion and  voluntary  movement  are  distinguished  as  the  func- 
tions of  animal  life.  In  all,  except  the  very  simplest  and 
minutest  creatures,  special  parts  or  groups  of  organs  are 
devoted  to  each  of  these  different  functions ;  while,  in  addi- 
tion, there  is  a  large  amount  of  structure,  whose  office  is  to 
give  protection  or  mechanical  support  to  the  rest  of  tho 
body. 

Nutrition  includes  the  various  processes  necessary  for  the 
growth  of  the  body  and  the  maintenance  of  its  substance. 
Every  living  part  of  every  living  being  undergoes  change  in 
the  particles  of  which  it  is  composed,  attracting  and  assimi- 
lating to  itself  materials  around  it,  and  parting  with  others 
which  undergo  decomposition ;  and  these  processes  of  waste 
and  repair  are  in  proportion  to  the  activity  of  the  part, 
every  manifestation  of  life  being  accompanied  with  chemical 
and  other  changes.  Thus  a  living  being  is  a  vortex,  the  par- 
ticles of  which  are  continually  changing,  while  the  form  con- 
tinues; and  vital  energy  is  a  force  correlative  with  mechanical, 
chemical,  and  other  forces  found  in  the  inorganic  world. 


14 


ANIMAL   PHYSIOLOGY. 


It  follows  from  this  that  every  part  of  a  complex  body, 
like  that  of  man,  requires  a  supply  of  nourishment  to  be 
brought  to  it,  and  a  channel  of  escape  for  waste  products ; 
and  to  meet  these  requirements  there  are  many  different 
organs.  The  immediate  source  of  nourishment  of  all  the 
tissues  is  the  blood,  but  to  recruit  the  supply  of  this  fluid 
new  materials  have  constantly  to  be  taken  in ;  and  to  fulfil 
this  end,  the  alimentary  canal  receives  the  food  and  digests 
it,  that  is  to  say,  reduces  the  newly  received  materials  to 
such  a  condition  that  they  can  be  taken  up  by  processes  of 
absorption  from  the  cavity  of  the  canal,  and  carried  to  the 
blood.  To  complete  the  elaboration  of  the  blood,  and  free  it 
from  impurities  constantly  resulting  from  waste,  a  variety  of 
organs  are  engaged,  of  which  the  principal  are  the  lungs, 
spleen,  liver,  kidneys,  and  skin.  Lastly,  to  convey  the 
blood  to  and  from  the  tissues  which  it  nourishes  and  the 
organs  in  which  it  is  purified,  a  system  of  ramifying  vessels 
is  required,  and  a  heart  with  muscular  force  sufficient  to 
propel  the  blood  through  them. 


Fig.  1.— -DIAGRAM  OF  CAPILLARY  NETWORK,  with  termination  of  an 
Artery  and  commencement  of  a  Vein. 

It  may  be  here  mentioned  that  the  system  of  blood-vessels 
is  completely  closed  in  all  vertebrate  animals,  the  blood 
being  distributed  from  the  heart  by  arteries,  circulating 
through  the  tissues  in  a  network  of  minute  vessels  called 
capillaries,  and  returning  to  the  heart  by  veins.  But  the 


FUNCTIONS   OP  ANIMALS.  15 

capillaries,  which  have  an  average  diameter  of  only  -^Vo  of 
an  inch,  have  walls  of  extreme  tenuity,  which  allow,  with 
the  utmost  freedom,  the  transition  of  materials  outwards  and 
inwards  between  them  and  the  surrounding  textures. 

Reproduction  is  the  function  by  means  of  which  new  in- 
dividuals are  developed  from  portions  of  pre-existing  living 
beings;  and,  although  it  is  not  generally  so  diffused  through- 
out the  body  as  the  function  of  nutrition,  but  is  delegated  to 
special  parts,  yet  it  may  be  remarked  that  in  many  low 
forms  of  animal  life,  and  some  even  of  the  higher  vegetable 
forms,  a  large  amount  of  reproductive  power  pervades  the 
entire  organism.  Also,  in  the  higher  animals  there  exists 
an  ill-understood  connection  between  the  reproductive  organs 
and  the  nutrition  of  the  body  generally,  which  is  of  a  two- 
fold description;  the  nutrition  of  the  body  being  importantly 
modified  by  the  condition  of  these  parts,  and  the  minute 
peculiarities  of  all  parts  of  the  body  being  capable  of  trans- 
mission to  the  offspring. 

Sensation  is  a  psychical,  and  not  a  physical  condition ; 
but  it  is  associated  and  bound  up  with  changes  in  the  body, 
the  seat  of  which  is  the  nervous  system.  The  mind  is  not 
influenced  by  external  objects,  save  when  these  irritate 
nerves  or  organs  of  sense  into  a  state  of  activity,  and  the 
active  condition,  travelling  along  nerve-trunks,  reaches  the 
part  of  the  brain  with  which  the  mind  is  specially  and  in- 
scrutably linked.  And  not  only  is  an  active  condition  of 
the  brain  necessary  to  influence  the  intelligence  by  external 
objects,  but  a  like  active^ condition  accompanies  all  emotion 
and  every  operation  of  the  mind. 

Movement  of  a  voluntary  description  is  accomplished  by 
muscles  receiving,  through  nerves,  their  stimulus  to  action 
from  the  brain,  which  in  turn  is  stimulated  in  an  unknown 
way  by  the  will.  Thus,  the  central  nervous  system  is  both 
the  terminus  to  which  messages  from  the  organs  of  sense  are 
sent,  and  that  from  which  commands  to  the  voluntary  muscles 
proceed. 

All  sensory  function,  however,  is  not  sensation,  and  all 
movement  is  not  voluntary.  The  nervous  system  may 
receive  an  influence  from  without,  and  transmit  it  to  groups 
of  muscles,  without  intervention  of  any  act  of  consciousness. 


16  ANIMAL   PHYSIOLOGY. 

This  is  what  is  called  reflex  action  (p.  17  8),  and  in  such  a 
case  the  part  irritated,  from  which  the  nervous  impulse 
starts,  is  still  said  to  have  sensibility,  and  the  nerve  to  be 
sensory,  although  there  is  no  sensation,  and  the  movement  is 
involuntary.  Also,  the  property  of  response  to  irritation  is 
not  confined  to  the  nervous  system ;  structures  may  alter 
their  shape  or  undergo  other  change  on  application  of  a 
stimulus,  and  this  property  is  termed  irritability.  The 
active  part  of  change  of  shape  or  movement  probably  in  all 
cases  consists  in  contraction,  and  is  hence  called  contrac- 
tility. 

Irritability  and  contractility,  although  they  may  well  be 
included  under  the  terms  sensory  function  and  movement, 
are  not,  like  sensation  and  voluntary  movement,  confined  to 
animals.  They  are  found  in  the  vegetable  world  also;  and 
it  may  be  maintained  with  probability,  that  they  are  proper- 
ties of  every  living  part  of  every  living  being. 

6.  The  expression  living  parts  of  living  beings,  has  been 
already  twice  used,  and  will  attract  the  student's  attention 
to  the  fact  that  every  part  of  the  texture  of  the  body  does 
not  equally  exhibit  the  phenomena  of  life.  In  a  large 
majority  of  the  different  textures,  a  considerable  or  even  the 
greater  part  of  the  bulk  is  composed  of  mere  deposited 
matter,  which,  although  it  undergoes  both  structural  and 
chemical  changes,  offers  no  sufficient  evidence  of  the  posses- 
sion of  properties  peculiar  to  living  beings;  but,  imbedded  in 
this,  or  in  other  instances  forming  the  principal  mass  of  the 
texture,  there  is  always  to  be  found  a  set  of  elements  which 
exhibit  some  or  all  of  the  four  functions — nutrition,  repro- 
duction, contractility,  and  irritability. 

These  living  elements  of  texture  always  consist  of  material 
belonging  to  one  chemical  group  of  substances ;  namely,  those 
which  are  termed  sometimes  the  proteids,  but  which  may 
probably  be  more  conveniently  distinguished  as  the  albumi- 
noids, albumen  and  fibrin  being  among  the  most  familiar 
examples  of  them.  The  substances  of  this  group  are  the 
most  complex  combinations  of  carbon,  hydrogen,  oxygen, 
and  nitrogen;  and,  as  they  are  found  in  nature,  contain 
also  phosphorus,  sulphur,  potash,  and  soda. 

Yery  frequently  the  expression  protoplasm  is  used  to  in- 


NUCLEATED  CORPUSCLES. 


17 


dicate,  without  much  definition,  the  varieties  of  albuminoid 
substance  found  in  the  growing  stages  of  the  living  elements 
of  texture,  and  in  the  lowest  forms  of  life. 

The  generalization  has  been  long  known,  and  may  be 
safely  made,  that  the  phenomena  of  life  are  never  exhibited 
without  the  presence  of  albuminoid  substance. 

The  simplest  form  of  living  element  in  both  animal  and 
vegetable  texture,  or  at  least  one  of  the  simplest  forms,  and 
the  most  important,  is  the  nucleated  corpuscle,  which  is 
remarkable  not  only  for  the  remarkable  part  which  it  plays, 
but  for  its  resemblance  to  some  of  the  simplest  kinds  of 
animals,  the  genus  Amoeba. 


Fig.  2. — SPECIES  OF  AMCEBA..     After  Pritchard. 

7.  Amoeba  is  the  name  of  a  family  of  animals  which  are 
microscopically  minute,  and  inhabit  both  salt  and  fresh  water. 
They  consist  of  a  mass  of  protoplasm  unlimited    by  any 
envelope,  containing  granules,  arid  usually  a  clear,  rounded, 
firmer  body,  the  nucleus,  with  a  still  denser  speck  in  its 
interior,  the  nucleolus.      This   mass    of  protoplasm  moves 
about  by  throwing  out  temporary  processes  in  different  direc- 
tions, and  changing  its  form  by  virtue  of  its  contractility. 
In  fact,  the  powers  of  assimilation,  reproduction,  irritability, 
and  contractility,  appear  all  to  be  present  in  one  common 
mass.     There  are  other  families  in  which  such  a  mass  as 
constitutes  the  amceba  is  surrounded  by  a  membranous  cover- 
ing or  a  hard  shell. 

8.  The  Nucleated  Corpuscles  found  in  the  ^textures  of  the 
higher  animals  present  many  varieties  of  appearance,  but  in 
their  young  and  active  condition  they  have  this  much  resem- 
blance to  amceba,  that  they  present  a  mass  of  protoplasm  with 
one  or  more  nuclei,  which  may  contain  nucleoli.     Some  are 
surrounded  with  a  membranous  envelope,  others  have  none, 
and  with  regard  to  a  great  number  of  them  it  is  extremely 
difficult  to  say  whether  they  have  a  membrane  round  them 

14  B 


18  ANIMAL   PHYSIOLOGY. 

or  not.  The  membrane,  when  present,  is  called  a  cell-wall^ 
and  the  structure  of  which  it  is  the  limit  is  a  nucleated  cell ; 
and  in  consequence  of  the  circumstance  that  the  mass  of  pro- 
toplasm was  the  last  part  of  the  corpuscle  to  have  due  atten- 
tion attracted  to  it,  and  that  the  outline  of  the  corpuscle  was 
often  mistaken  for  a  membrane,  even  when  no  membrane 
existed,  the  importance  of  the  cell-wall  was  formerly  over- 
estimated, and  the  word  cell  is  even  yet  often  used  to  indicate 
structures  without  a  cell-wall,  which  are  better  designated  as 
corpuscles.  The  cell-wall  is  probably  in  all  instances  a  deposit 
round  a  pre-existing  corpuscle. 

At  an  early  period  of  embryonic  existence,  the  body  may 
be  said  to  consist  entirely  of  nucleated  corpuscles;  and  even 
after  birth,  the  younger  the  animal  the  more  abundant  are 
these  elements  in  the  textures,  and  the  more  easily  exhibited 
under  the  microscope.  They  are  found  in  numbers  wherever 
there  is  much  growth;  and  in  rapidly  increasing  tumours 
they  exist  in  greatest  plenty.  They  are  also  the  germs  from 
which  the  more  complex  elements  of  texture  take  origin. 
Thus  nerves  and  voluntary  muscular  fibres  originate  by 
metamorphosis  of  nucleated  corpuscles,  which,  in  becoming 
more  highly  developed,  lose  the  reproductive  power,  while 
they  gain,  in  the  one  case,  nervous  activity,  and  in  the  other 
greatly  increased  contractility.  Both  of  these  tissues  in  early 
development  present  long  bands  of  albuminoid  substance, 
with  a  row  of  nuclei  in  each. 

B 


Fig.  3. — MULTIPLICATION  OF  NUCLEATED  CORPUSCLES.  A,  Corpuscles 
from  connective  tissue  of  a  foetal  lamb,  some  of  them  dividing.  B, 
Endogenous  multiplication  within  a  brood  cell  from  a  tumour. 

Nucleated  corpuscles  multiply  by  division,  which  is  termed 
fissiparous  when  the  parts  into  which  they  divide  are  of 


NUCLEATED  CORPUSCLES.  19 

similar  magnitude,  gemmiparous  when  buds  are  separated 
from  a  parent  mass,  as  occurs  frequently  in  vegetables.  When 
a  corpuscle  divides  within  a  cell-wall,  which  remains  unrup- 
tured,  the  process  of  multiplication  is  called  endogenous.  The 
nucleus  seems  to  play  an  important  part  in  the  multiplica- 
tion of  corpuscles,  being,  at  least  in  many  instances,  the  first 
part  to  divide. 

At  present  the  weight  of  evidence  appears  to  be  in  favour 
of  every  corpuscle  being  derived  from  a  parent.  Certain 
physiologists  hold  a  contrary  opinion;  but  there  is  no  well 
determined  instance  of  these  structures  originating  otherwise 
within  the  body. 

The  varieties  of  nucleated  corpuscles  found  in  different 
situations  will  come  under  notice  in  the  description  of  the 
individual  textures. 


CHAPTER  II. 
THE  CONNECTIVE  TISSUES. 

9.  THE  most  widely  distributed  texture  in  the  body  is  that 
which  is  termed  connective  tissue.  It  is  the  substance  which 
connects  the  integument  everywhere  with  the  deeper  struc- 
tures, and  it  makes  partitions  between  these  structures  and 
between  the  elements  of  which  they  are  composed.  Thus 
every  muscle  has  a  filmy  sheath,  which,  when  separated,  is 
seen  to  consist  of  a  felted  white  substance,  sending  in  pro- 
cesses into  the  muscle,  dividing  its  substance  into  bundles, 
and  these  into  still  smaller  bundles  by  finer  investments. 
The  same  substance  separates  the  bundles  of  every  nerve, 
and  surrounds  the  blood-vessels;  it  is  found  in  great  quan- 
tity among  fat  and  beneath  the  integument,  and  it  forms  in 
fact  a  continuous  web  in  which  all  the  structures  throughout 
the  body  are  imbedded. 

This  tissue,  looked  at  with  the  microscope,  exhibits  two 
elements :  first,  a  matrix,  which,  in  those  places  where  it  is 
most  closely  mixed  up  with  other  textures,  is  often  homo- 
geneous or  nearly  so,  but  which,  in  the  denser  specimens 
obtained  from  distinct  masses,  assumes  the  appearance  of 
extremely  fine  fibres  of  indefinite  length,  disposed  in  irre- 
gular felted  fashion,  leaving  spaces,  from  which  the  tissue 
gets  the  name  areolar ;  secondly,  nucleated  corpuscles, 
called  in  this  instance  connective-tissue-corpuscles. 

A  drop  of  dilute  acetic  acid  added  to  the  specimen  under 
the  microscope,  causes  the  fibres  of  the  matrix  to  'swell  up  and 
become  indistinct,  bringing  the  nuclei  of  the  corpuscles  clearly 
into  view,  and  also  a  variable  admixture  of  isolated  fibres 
on  which  the  acid  has  no  effect.  The  fibres  on  which  acetic 
acid  has  no  action  are  called  elastic  fibres,  and  will  be  further 
referred  to ;  those  which  are  swollen  up  by  the  acid  are  called 


CONNECTIVE   TISSUES. 


21 


white  fibres,  and  constitute  the  bulk  of  all  white  fibrous 
tissue.  White  fibres  become  completely  dissolved  by  pro- 
longed boiling,  being  converted  into  gelatin,  a  nitrogenous 


Fig.  4.— CONNECTIVE  TISSUE  FROM  THE  ORBIT  OF  THE  Ox,  exhibit- 
ing corpuscles  of  various  shapes,  felted  white  fibres,  and  a  few 
slightly  curled  elastic  fibres. 

substance  of  simpler  chemical  constitution  than  the  albumi- 
noids, and  characterized  by  dissolving  in  hot  water  and  form- 
ing a  jelly  on  cooling.  Albuminoid  textures,  on  the  contrary, 
are  coagulated  by  boiling ;  and  thus  it  is  that  when  meat  is 
boiled,  the  flesh  or  muscular  fibre  is  hardened,  while  the  con- 
nective tissue  between  the  muscular  fibres  is  softened  and 
ultimately  dissolved.  This  cannot  be  illustrated  better  than 
by  comparing  a  raw  fish  with  one  which  has  been  cooked. 
In  the  raw  fish  the  semi-transparent  and  comparatively  soft 
segments  of  muscle  are  united  by  firm  septa,  tough  and 
strong;  in  the  cooked  state  they  are  opaque  and  hard,  but 
fall  separate,  because  the  septa  dissolve  into  gelatin. 

The  connective-tissue-corpuscles,  being  of  albuminoid  sub- 
stance, resist  boiling,  and  their  examination  is  sometimes 
facilitated  by  that  means.  They  often  present  a  stellate 
appearance,  sending  out  branches  or  processes  in  different 


22  ANIMAL   PHYSIOLOGY. 

directions.  ID  the  web  of  the  frog's  foot,  and  in  other  trans- 
parent textures  capable  of  being  examined  microscopically 
in  living  animals,  they  have  been  seen  not  only  changing 
their  shapa  but  even  moving  about,  so  that  they  may  well 
be  termed  amoeboid.  In  the  fine  interstices  between  other 
tissues,  nuclei  are  often  seen  in  great  abundance  in  homo- 
geneous matrix,  without  any  apparent  protoplasm  about 
them. 

It  may  be  here  mentioned,  that  to  bring  delicate  textural 
elements  such  as  connective-tissue-corpuscles  into  view  under 
the  microscope,  many  niceties  of  method  are  resorted  to,  and 
among  these  there  are  some  points  which  deserve  special 
attention.  The  material  should  be  perfectly  fresh,  and  not 
allowed  to  come  in  contact  with  water,  as  water  swells  up 
and  destroys  delicate  corpuscles.  Spirit,  on  the  other  hand, 
shrivels  textures.  By  using  serum  and  various  weak  solu- 
tions, these  deleterious  effects  are  avoided.  Principal  among 
preservative  substances,  weak  solutions  of  chromic  acid  and 
bichromate  of  potash  may  be  mentioned,  to  which  spirit  may 
be  daily  added  in  small  quantities.  "Water  added  to  specimens 
previously  treated  with  chromic  acid  no  longer  destroys  the 
corpuscles.  In  examining  nucleated  corpuscles,  staining  with 
an  ammoniacal  solution  of  carmine  is  often  of  the  greatest 
service :  the  specimen  should  be  washed  after  being  stained, 
and  should  then  be  put  up  for  the  microscope  in  glycerine. 
Yery  often  the  beauty  of  the  specimen  is  greatly  increased 
by  addition,  after  glycerine,  of  a  little  nitric  acid.  This 
must,  however,  be  carefully  washed  away  again,  before  it  has 
had  time  to  destroy  the  carmine  staining. 

10.  White  Fibrous  Tissue. — The  term  connective  tissue 
is  a  very  general  one,  and  the  varieties  to  which  we  have 
already  referred  are  the  homogeneous  and  areolar;  but  there 
are  others  which  are  more  markedly  fibrous,  and  constitute 
the  group  of  white  fibrous  tissues,  namely,  fascia,  aponeurosis, 
tendon,  and  ligament. 

Fascia  is  the  name  given  to  strong,  felted  arrangements  of 
white  fibrous  tissue  spread  out  in  sheets.  An  aponeurosis 
is  a  sheet  of  white  fibrous  tissue  arranged  in  parallel  fasciculi, 
or  in  two  or  more  sets  of  decussating  fasciculi,  and  having  in 
consequence  a  shining  appearance.  Tendon  is  white  fibrous 


ELASTIC   TISSUE. 


23 


tissue  used  for  the  attachment  of  muscles,  and  may  either  be 
arranged  in  the  form  of  aponeurosis  or  in  solid  bands ;  it  has 
a  satin-like  lustre  of  con- 
siderable brilliancy.  When 
tendons  rupture  during  life, 
they  snap  straight  across. 
Ligament  is  white  fibrous 
tissue     used    for    binding 
bones  together;    it  is  not 
so  lustrous  as  tendon ;  and 
when  it  gives  way,  which 
is  very  seldom,  it  does  not 
snap  but  tears.    In  all  these 
fasciculated   forms  of  con- 
nective   tissue,    the     cor- 
puscles are  elongated,  and 
lie  in  the  direction  of  the      Fig.  5.— APONEUROSTS,  human, 
fibres.     In  tendon  they  are  flattened,  and  have  been  seen 
in  specimens  from  young  animals  to  lie  in  sheets  between 
bundles  of  the  fibrous  substance.     Tendon  and  ligament  are 
quite  inextensible,  and  all  white  fibrous  tissue,  even  when  by 
injury  to  its  texture  it  is  gradually  stretched,  is  destitute  of 
resiliency. 

11.  Elastic  Tissue  occurs  both  in  the  form  of  fibres  and 
thin  homogeneous  membranes.  It  gets  its  name  from  being 
highly  extensible  and  resilient,  and  is  most  widely  distributed 
in  the  fibrous  form.  In  the  human  subject  there  is  only  one 
set  of  ligaments  which  consist  of  nearly  pure  elastic  tissue, 
namely,  the  ligamenta  subflava,  which  join  together  the 
arches  of  the  vertebrae,  and  get  their  name  from  the  yellow 
colour  peculiar  to  elastic  fibres.  They  facilitate,  by  their 
resiliency,  the  resumption  of  the  erect  posture  when  the  back 
has  been  bent  forward.  In  quadrupeds  two  other  notable 
instances  of  pure  elastic  tissue  may  be  mentioned.  One  is 
the  ligamentum  nuchse,  a  strong  band  extending  from  the 
back  of  the  skull,  and  attaching  it  to  the  withers  or  dorsal 
spines;  the  other  is  in  the  form  of  an  aponeurosis,  which 
lies  on  the  abdominal  wall,  and  aids  the  support  of  the 
viscera.  On  examining  fibres  from  any  of  these  sources,  it  is 
seen  that  they  may  be  of  considerable  breadth;  that  loosened 


24 


ANIMAL   PHYSIOLOGY. 


from  their  attachments  and  teased  out  they  curl  up  afc  the 
ends,  that  they  refract  light  very  strongly,  and  that  they  are 
unaltered  by  acetic  acid.  Elastic  tissue  is  not  easily  altered 


Fig.  6.— ELASTIC  TISSUE.     A,  From  ligeamentum  nuchse  of  sheep. 
B,  From  pleural  surface  of  lung. 

by  even  prolonged  boiling,  and  yields  no  gelatin.     In  many 
places,  as,  for  example,  underneath  the  pleura,  isolated  elastic 
fibres  are   exceedingly  abundant,   of  great   length,   curling 
naturally,  and  crossing  one  another  in  all  directions. 
A    /^  3 


Fig.  7. — ADIPOSE  TISSUE.  A,  The  usual  appearance  of  fat-cells ; 
a,  shows  a  nucleus  on  the  side  of  a  fat-cell;  b,  cell 'filled  partly 
with  water,  partly  with  oil.  B,  Scheme  of  the  mode  of  accumu- 
lation of  oil  in  young  fat-cells.  •„ 

12.  Adipose  Tissue  is  the  term  technically  used  for  the 
fat  of  the  body,  because  fat  in  its  proper  acceptation  means  a 
solid  oil,  such  as  tallow.  Adipose  tissue  consists  of  a  number 


ADIPOSE   TISSUE.  25 

of  minute  vesicles,  varying  in  diameter  up  to  -^^  of  an  inch, 
filled  with  oil,  and  imbedded  in  groups  in  connective  tissue. 
Sometimes  a  nucleus  can  be  detected  at  the  side  of  the  vesicle. 
The  mode  of  development  appears  to  be  that  one  or  more 
minute  globules  of  oil  occur  at  first  in  the  interior  of  a  con- 
nective-tissue-corpuscle, and  that  the  oil  goes  on  accumulating, 
pushing  before  it  the  substance  of  the  corpuscle,  which  sub- 
sequently is  so  altered  in  appearance  and  consistence,  as  to 
form  the  wall  of  the  vesicle  or  adipose  cell.  Adipose  tissue, 
and  the  connective  tissues  o-enerally,  are  but  scantily  sup- 
plied with  blood-vessels 


ANIMAL   PHYSIOLOGY. 


j?igt  8.— THE  SKELETON. 


CHAPTER  III. 
THE   SKELETON. 

13.  BY  the  skeleton  is  meant  the  hard  framework  of  the 
body.     It  consists  of  bones,  cartilages,  and  ligaments. 

What  is  called  the  backbone,  or,  more  properly,  the  verte- 
bral or  spinal  column,  may  be  said  to  be  the  central  part  of 
the  skeleton.  It  is  composed  of  a  series  of  bones  called 
vertebra,  the  fore  parts  or  bodies  of  which,  united  by  means 
of  discs  of  flexible  tissue,  constitute  a  pillar  of  support,  while 
what  are  termed  the  arches,  lying  behind  this  pillar,  form  a 
protective  cylinder  round  the  spinal  cord,  have  spinous  and 
transverse  processes  projecting  from  them,  and  glide  one  on 
another  by  joints.  There  are  twenty-four  of  these  movable 
vertebrae,  the  seven  highest  of  which,  belonging  to  the  neck, 
are  called  cervical,  while  the  following  twelve  carry  ribs  and 
are  called  dorsal,  and  the  remaining  five  are  termed  lumbar. 
They  are  succeeded  by  the  sacrum  and  coccyx,  which  form 
the  lower  part  of  the  vertebral  column,  and  will  be  further 
alluded  to. 

Springing  from  the  dorsal  portion  of  the  vertebral  column 
are  twelve  pairs  of  ribs,  which  are  further  prolonged  in  front 
by  means  of  costal  cartilages.  The  costal  cartilages  of  the 
upper  seven  pairs  of  ribs  are  prolonged  forwards  to  the  breast- 
bone or  sternum,  to  be  fitted  into  its  sides;  those  of  the  suc- 
ceeding five  pairs  are  each  fixed  to  the  cartilage  next  above; 
while  those  of  the  eleventh  and  twelfth  ribs  are  pointed,  and 
terminate  in  the  muscular  wall  of  the  abdomen.  The  circles 
formed  by  the  ribs  and  parts  with  which  they  are  connected 
are  called  costal  arches,  while  the  series  of  ribs  and  costal 
cartilages,  together  with  the  dorsal  vertebrae  and  sternum, 
constitute  the  thorax  of  the  skeleton. 

14.  Articulating  with  the  upper  end  of  the  sternum,  in  tho 


28 


ANIMAL  PHYSIOLOGY. 


human  subject,  are  the  collar  bones  or  clavicles,  which  imite 
the  shoulders  with  the  skeleton  of  the  trunk.  The  clavicle 
has  no  existence  in  many  mammals,  such 
as  the  horse,  the  ox,  and  the  sheep ; 
while  in  others  it  is  rudimentary  and 
without  function,  as  in  the  cat  and  the 
dog;  and  in  all  such  instances  the  shoulder 
and  fore  limb  are  united  to  the  rest  of  the 
skeleton  by  mere  muscular  connections; 
but  in  the  animals  in  which  it  exists — foi 
example,  squirrels  and  monkeys — it  is  the 
fulcrum  on  which  the  arm  moves  when 
stretched  out  from  the  body  or  approached 
to  the  middle  line.  The  outer  end  of  the 
clavicle  articulates  with  the  scapula  or 
shoulder-blade,  and  the  two  bones  together 
form  the  shoulder-girdle.  '•* 

The  joints  at  the  outer  and  inner  ends 
of  the  clavicles  permit  the  shoulder-blades 
to  be  moved  upwards,  downwards,  for- 
wards, or  backwards  at  will,  while  they 
continue  to  glide  on  the  conical  walls  of 
the  upper  part  of  the  chest.  The  part 
of  the  shoulder-blade  with  which  the 
clavicle  articulates  is  called  the  acromion, 
and  is  the  expanded  extremity  of  a  spine 
which  arises  from  the  back  of  that  bone, 
and  is  directed  outwards  and  upwards. 
At  a  little  distance  from  its  outer  end, 
the  clavicle  is  likewise  united  by  strong 
ligaments  to  another  process  of  the 
shoulder-blade  called  the  coracoid,  against 
which  it  rests  when  the  shoulders  are 
pushed  upwards. 

The  humerus  or  arm  bone  articulates 
by  a  rounded  head  with  a  surface  of  the 
scapula  called  the  glenoid  fossa,  distinct 
Fig.  9.— VERTEBRAL  from  both  acromion    and   coracoid   pro- 
COLUMN.  cesses,  and  this  articulation  is  the  shoulder 

joint.     It  permits  greater  freedom  of  motion  than  any  other 


THE   SHOULDER. 


29 


joint  in  the  body,  owing  to  the  smallness  of  the  scapular  sur- 
face, compared  with  the  globular  humeral  surface,  on  which 


Fig.  10. — SHOULDER,     a,  clavicle;  b,  acrornion;  c,  coracoid  process ; 
d,  glenoid  fossa  of  the  scapula;  e,  humerus. 

it  moves,  and  the  looseness  of  the  liga- 

mentous  capsule  which  unites  the  two 

bones;  but  the  coracoid  and  acromial 

processes  overhang  the  joint  sufficiently 

to  add  greatly  to  its  strength;  for  it 

is  against  them  that  the  humerus  is  in 

great  measure  pushed  in  all  positions  in 

which  great  pressure  is  made  against  it.- 
In  the  forearm  there  are  two  bones 

named  radius  and  ulna.    The  ulna,  the 

inner  of  the  two,  is  strong  above  and 

slender  below,  and  admits  of  no  move- 
ment save  in  a  hinge  fashion  on  the 

humerus,  with  which  it  articulates  by 

means  of  a  cavity  which  looks  forwards, 

and  is  bounded  below  by  the  coronoid 

process,  above  by  the  olecranon  or  pro- 
minence  of  the   elbow.      The    radius,  Fig.  11. 

Fig.  11. — RADIUS  AND  ULNA,  a,  olecranon  process  of  ulna;  5, 
coronoid  process;  c,  orbicular  ligament,  embracing  the  head 
of  the  radius ;  d,  triangular  ligament,  uniting  the  radius  to 
the  styloid  process  of  the  ulna. 


30 


ANIMAL  PHYSIOLOGY. 


which  is  much  slenderer  above  than  at  its  lower  end,  and 
supports  the  hand,  is  bound  to  the  ulna  at  its  upper  end 
by  an  orbicular  ligament,  which  permits  it  to  rotate  with- 
in its  grasp,  and  is  fastened  to  it  below  in  such  a  way 
that  it  revolves  round  that  bone  as  on  a  pivot,  carry- 
ing with  it  the  hand,  and  accomplishing  pronation  and 
gupination,  or  the  turning  of  the  palm  downwards  and 
upwards. 

The  hand  consists  of  eight 
little  carpal  bones  arranged 
in  two  rows,  five  metacarpal 
bones,  which  form  the  skeleton 
of  the  palm,  and  the  phalanges 
or  finger  bones,  of  which  the 
thumb  has  two,  and  the  other 
digits  three  each. 

The  bones  of  the  upper  row 
of  the  carpus  are  named — 
beginning  at  the  outer  or 
thumb  side  —  the  scaphoid, 
semihmar,  and  cuneiform 
bones,  and  the  pisiform, 
smaller  than  these,  and  arti- 
culated in  front  of  the'  cunei- 
form. The  bones  of  the  second 
row  are  called  trapezium,  tra- 
pezoid,  os  magnum,  and  unci- 
form, the  unciform  supporting 


Fig.  12. — FKONT  VIEW  OF  THE 
BONES    OF     THE    HAND,      a, 


the  metacarpal  bones  of  the 
ring   and  little   fingers,   and 


trapezium;    6,  scaphoid,  _and,      the-  others     Supp0rting    one 


beneath  it,  the  trapezoid;  c, 
semilunar,  and,  beneath  it,  os 
magnum;  d,  pisiform;  e,  cunei- 
form; /,  unciform. 


metacarpal  bone  each. 

The  movements  of  the  wrist 

are  accomplished  partly  by 
movement  of  the  upper  row  of  carpal  bones  on  the  radius, 
and  partly  by  one  row  of  carpal  bones  moving  on  the  other. 
There  is  little  perceptible  movement  allowed  between  the 
carpal  bones  of  the  second  row;  but  it  is  not  without  import- 
ance that  they  are  separate  bones;  for  when  we  lean  or  push 
with  the  palm,  and  the  wrist  is  over-extended,  the  members 


THE   HAND.  31 

of  tliis  range,  as  well  as  the  metacarpal  bones,  present  the 
concavity  of  an  arch  towards  the  object  pressed  on,  and  have 
the  ligaments  which  support  them  in  that  position  thrown 
into  a  state  of  tension,  which,  being  recovered  from  as  soon 
as  the  pressure  is  removed,  gives  elasticity  to  the  movements 
of  the  limb.  The  utility  of  the  hand  depends  in  great 
measure  on  the  opposability  of  the  thumb  to  the  other  digits, 
and  this  results  from  freedom  of  movement  between  the 
trapezium  and  first  metacarpal  bone,  and  from  the  number 
of  muscles  attached  to  the  thumb.  - 

15.  The  fifth  or  lowest  lumbar  vertebra  rests  on  the  broad 
upper  end  of  a  curved  wedge,  the  sacrum,  which  consists  of 
five  other  vertebra  fused  together  in  one  bone;  and  at  the 
lower  and  narrow  end  of  this  bone  are  four  more  of  a  rudi- 
mentary description,  corresponding  with  the  caudal  vertebrae 
or  bones  of  the  tail  in  other  animals,  but  usually  named  by 
the  human  anatomist,  collectively,  the  coccyx. 

On  its  sides,  in  the  upper  two-thirds  of  its  extent,  the 
sacrum,  is  closely  united  to  the  two  pelvic  or  innominate 
bones,  which,  together  with  it,  enclose  a  basin  or  cavity, 
called  the  pelvis.  Examined  in  early  life,  each  innominate 
bone  is  seen  to  consist  of  three  parts,  which  meet  at  the 
articular  cup,  called  the  acetabulum,  for  the  head  of  the 
thigh  bone.  The  expanded  upper  part  is  called  the  ilium, 
the  lower  part  is  called  the  ischium,  while  tthe  part  which 
meets  with  the  opposite  bone  in  the  middle  line  is  the  os 
pubis,  and  the  union  is  called  the  symphysis  pubis.  The 
expanded  ilium  obviously  corresponds  with  the  shoulder- 
blade  in  the  upper  limb,  and  the  pair  of  innominate 
bones  with  the  shoulder-girdle,  notwithstanding  that  the 
shoulder-girdle  is  but  little  connected  with  the  trunk,  while 
the  innominate  bones  take  an  important  part  in  bounding 
the  visceral  cavity. 

From  the  upper  end  of  the  sacrum  a  prominent  ring  of 
the  innominate  bone  can  be  followed  round  to  the  symphysis 
pubis,  constituting  the  brim  of  the  true  pelvis  as  distin- 
guished from  the  part  of  the  abdomen  between  the  blades  oi 
the  iliac  bones.  In  the  erect  posture  of  the  body  this  ring 
lies  at  an  angle  of  60°  with  the  horizontal,  so  that  the 
sacrum,  presses  downwards  on  it.  But  the  sacrum  is  so 


32  ANIMAL   PHYSIOLOGY. 

i 

placed  that  its  upper  end  is  thrown  in  front  of  the  rest  of  it, 
and  that  its  hinder  surface,  which  is  narrower  than  the 
anterior,  looks  upwards;  it  would,  therefore,  fall  down  into 
the  pelvis  were  it  not  supported  by  a  pair  of  exceedingly 
strong  posterior  sacro-iliac  ligaments ;  and  it  is  through 
these  ligaments  much  more  than  by  direct  pressure  that 
the  weight  of  the  body  is  conducted  from  the  sacrum  to 
the  pelvis. 


Fig.  13. — SECTION"  OF  PELVIS,  showing  the  suspension  of  the  sacrum 
between  the  haunch  bones,     a,  the  posterior  sacro-iliac  ligaments. 

16.  The  thigh-bone  or  femur  corresponds  with  the  humerus 
of  the  upper  limb ;  in  front  of  the  knee  is  the  patella  or  knee- 
cap, which  is  a  sesamoid  bone  or  ossification  within  a  tendon, 
and  not  at  all  correspondent  with  the  olecranon  of  the  elbow : 
in  the  leg  the  tibia  and  fibula  are  the  bones  corresponding 
with  the  radius  and  ulna  in  the  fore-arm;  and  in  the  foot 
there  is  a  close  correspondence  of  all  the  bones  with  those  of 
the  hand. 

The  parts  of  the  foot  are  called  the  tarsus,  metatarsus,  and 
phalanges.  The  phalanges  and  metatarsal  bones  are  arranged 
quite  like  those  of  the  hand,  the  great  toe  being  similar  to  the 
thumb  in  having  only  two  phalanges.  The  bones  behind  the 
metatarsals  are  the  internal,  middle,  and  external  cuneiform 


THE   FOOT. 


33 


bones,  supporting  the  three  inner  metatarsals,  and  the  cuboid, 
supporting  the  fourth  and  fifth. 
These  four  bones  obviously  corre- 
spond with  the  four  carpal  bones  of 
the  lower  range  in  the  hand  :  but 
behind  them  are  three  others,  the 
scaphoid,  lying  behind  the  three 
cuneiform  bones ;  the  calcaneum,  a 
very  large  bone  projecting  back  from 
the  cuboid,  and  forming  the  heel ; 
and  the  astragalus,  resting  on  the 
calcaneum  behind,  pressing  against 
the  scaphoid  in  front,  and  articulat- 
ing with  the  leg-bones  above  ;  and 
the  dissimilarity  of  appearance  of 
the  tarsus  as  compared  with  the  car- 
pus, is  due  to  the  large  and  unequal 
development  of  these  three  bones. 
The  scaphoid,  however,  corresponds 
with  the  bone  of  the  same  name  in 
the  hand,  and  the  astragalus  with 
the  semilunar,  while  the  calcaneum 
represents  the  cuneiform  and  the 
pisiform  together;  and  it  is  owing 
to  the  great  development  backwards 
of  the  calcaneum  to  form  the  heel, 
that  the  hollow  by  which  tendons 
and  other  structures  pass  from  the 
back  of  the  leg  to  the  sole  of  the 
foot  is  turned  inwards  so  as  to 
lie  between  the  heel  and  inner 
ankle. 

The  same  principle  of  conduction 
of  pressure  through  tense  ligaments, 
which  we  have  noted  in  the  hand  and 
the  pelvis,  is  resorted  to  again  in  the  foot.  The  foot  may  be 
conveniently  regarded  as  consisting  of  two  arches  supported 
behind  by  a  common  pier,  the  back  part  of  the  calcaneum. 
To  the  inner  and  principal  arch  belong  the  three  inner  toes, 
and  the  keystone  of  this  is  the  astragalus,  the  fore  part  or 
H  "  c 


g.  14. — FOOT  from  be- 
low, a,  calcaneum;  b, 
astralagus;  c,  scaphoid ; 

d,  internal  cuneiform; 

e,  cuboid.    The  middle 
and  external  cuneiform 
are  seen  between  the 
cuboid     and    internal 
cuneiform. 


34  ANIMAL  PHYSIOLOGY. 

head  of  which,  lying  between  the  calcaneum  and  scaphoid, 
is  retained  in  position  by  a  strong  inferior  calcaneo-scaphoid 
ligament,  which  has  frequently  to  bear  nearly  the  whole 
weight  of  the  body.  The  outer  arch  is  continued  forwards 
from  the  calcaneum  to  the  cuboid  and  two  outer  toes,  and  is 
prevented  from  falling  flat  by  strong  calcaneo-cuboid  or 
plantar  ligaments. 


Fig,  15. — SECTION    OF    FOOT,    showing,    a,   the   inferior  calcaneo- 
scaphoid  ligament  supporting  the  head  of  the  astralagus. 

17.  The  first  and  second  cervical  vertebrae  are  termed  the 
atlas  and  axis,  and  are  specially  modified  to  facilitate  move- 
ments of  the  head,  which  rests  on  them.  The  atlas,  instead 
of  presenting  a  body  in  front  and  an  arch  behind,  has  the 


Fig.  16. — ATLAS  AND  Axis.  A,  Upper  surface  of  atlas;  above  is 
the  ring  for  the  spinal  cord,  and,  separated  from  it  by  the  trans- 
verse ligament,  is  the  ring  for  the  odontoid  process  below :  to 
the  sides  of  this  are  the  surfaces  which  articulate  with  the  skull. 
B,  Front  view  of  the  axis,  with  the  arch  seen  in  perspective 
behind  the  odontoid  process. 

hollow  of  its  arch  prolonged  forwards,  between  the  articular 
portions  which  carry  the  skull ;  and  in  the  recent  state,  the 
anterior  part  of  this  hollow  is  converted  into  a  separate  ring 
by  a  transverse  ligament.  Through  this  anterior  ring  pro- 


THE  SKULL.  35 

f 

jects  a  process  which  surmounts  the  body  of  the  axis,  namely, 
the  odontoid  process,  and  round  this  the  atlas,  carrying  with 
it  the  skull,  revolves  as  on  a  pivot.  The  motion,  however, 
is  limited  by  two  lateral  bands,  the  check  ligaments,  which 
pass  out  from  the  top  of  the  odontoid  process  to  be  attached 
to  the  sides  of  the  foramen  magnum,  the  opening  in  the 
skull  by  which  the  cranial  cavity  is  made  continuous  with 
that  of  the  spinal  column.  By  the  study  of  development  and 
the  anatomy  of  different  animals,  it  is  well  ascertained  that 
the  odontoid  process  is  really  the  body  of  the  atlas,  which 
has  become  fastened  to  the  top  of  the  body  of  the  axis,  and 
remained  separated  by  a  joint  from  the  other  parts  of  the 
bone  to  which  it  in  one  sense  belongs. 

18.  The  skull  consists  of  cranium  and  face.  The  cranium, 
or  part  enclosing  the  brain,  is  counted  as  having  eight  bones. 
But  it  is  right  that  even  a  tyro  should  understand  that 
various  of  these  bones  consist  of  different  elements  which 
have  become  fused  together  at  an  early  age,  while  some  of 
what  are  considered  as  distinct  bones  are  also  fused  together 
in  every  adult.  The  word  "  bone"  is,  therefore,  used  some- 
what arbitrarily  in  speaking  of  the  bones  of  the  skull.  The 
occipital  bone  forms  part  both  of  the  base  and  the  roof,  and 
is  pierced  by  the  foramen  magnum.  The  other  bones  of  the 
roof  are  the  two  parietals  and  the  frontal,  which  in  the  child 
is  divided  down  the  middle  like  the  parietals.  The  frontal 
not  only  forms  a  large  part  of  the  vault  of  the  skull,  but  also 
the  roofs  of  the  orbits  or  sockets  of  the  eyeballs.  In.  the 
base  of  the  skull,  a  complex  bone,  the  sphenoid,  formed  by 
the  junction  of  many  elements,  and  primarily  divisible  into 
an  anterior  and  posterior  part  which  are  distinct  in  most 
animals,  extends  forwards  from  the  occipital,  with  w^hich  it 
is  thoroughly  united  in  the  adult,  and  reaches  the  orbital 
plates  of  the  frontal ;  while  the  interval  between  the  orbits 
is  filled  in  by  the  upper  part  of  a  delicate  and  .likewise  com- 
plex bone,  the  ethmoid,  which  is  pierced  with  foramina  for 
the  filaments  of  the  nerves  of  smell,  and  takes  much  greater 
part  in  the  formation  of  the  cavity  of  the  nose  than  in  com- 
pleting the  cranial  walls. 

Lastly,  on  the  sides  of  the  skull,  and  projecting  into  its 
base  between  the  sphenoid  and  occipital,  are  the  temporal 


36 


ANIMAL   PHYSIOLOGY. 


bones,  which  contain  the  organs  of  hearing  in  their  interior. 
The  part  of  the  temporal  which  lies  above  the  external  audit- 
ory meatus,  or  opening  of  the  ear,  is  termed  the  squamous 
portion;  the  thick  process  behind  is  called  mastoid;  the 
pyramidal  projection  into  the  base  is  the  petrous  portion; 
and  a  plate  which  forms  the  inferior  limit  of  the  opening  of 
the  ear,  and  of  the  cavity  into  which  it  leads,  is  the  tympanic 
plate.  Enclosed  by  this  plate,  within  what  is  called  the 
tympanic  cavity,  are  three  little  ossicles,  which  will  be  de- 
scribed with  the  organ  of  hearing. 


Fig.  17.  —  SKULL.  A,  Profile  view.  B,  Vertical  section,  a, 
occipital  bone;  &,  parietal;  c,  frontal;  d,  squamous  portion  of 
temporal ;  e,  mastoid  portion ;  /,  petrous  portion ;  gt  sphenoid  ; 
h,  pterygoid  process  of  sphenoid ;  i,  ethmoid ;  &,  nasal ;  ly 
superior  maxillary ;  m,  pre -maxillary  part  of  superior  maxillary ; 
n,  palatal;  o,  malar;  p,  lachrymal;  q,  inferior  maxillary;  r, 
inferior  turbinated.  _,  -  . 

19.  Of  the  face  bones,  the  largest  is  the  inferior  maxilla,  or 
lower  jaw,  and  this  is  the  only  one  which  is  movably  artic- 
ulated. The  remaining  part  of  the  face  consists  mainly  of 
the  walls  of  a  passage,  the  interior  of  which  is  divided  into 
the  right  and  left  nasal  fossae  by  a  mesial  bone  called  the 
vomer,  together  with  a  mesial  plate  of  the  ethmoid.  The 
iloor  of  this  passage  constitutes  the  palate.  Much  the  larger 
part  of  this  division  of  the  face  in  the  human  subject  is 
formed  by  the  superior  maxillary  bones,  which  carry  all 
the  upper  teeth,  and  represent  two  pairs  of  bones  in  the 


PECULIARITIES  OF  THE  HUMAN  SKELETON.  37 

skulls  of  other  mammals,  namely,  the  superior  maxillaries 
and  the  pre-maxillaries.  Behind  the  superior  maxillaries 
are  the  palatals,  which  form  the  back  part  of  the  palate,  and, 
by  means  of  ascending  portions,  join  together  the  superior 
maxillaries  and  what  are  called  the  pterygoid  processes 
of  the  sphenoid  bone,  two  processes  projecting  downwards 
from  the  base  of  the  skull.  It  may  also  be  mentioned  that 
the  inner  parts  of  these  processes,  namely,  the  internal  ptery- 
goid plates,  are  separate  bones  in  most  animals.  The  superior 
maxillaries  send  up  a  pair  of  processes  to  the  frontal  bone, 
behind  the  two  nasals,  the  bones  forming  the  ridge  of  the 
nose ;  but  they  get  a  much  stronger  support  from  a  pair  of 
cheek  bones,  the  jugals  or  molars,  which  project  outwards 
from  them,  and  each  of  which  sends  one  process  up  to  the 
frontal  to  complete  the  outer  wall  of  the  orbit,  and  another 
backwards  to  form  an  arch  with  what  is  called  the  zygomatic 
process  of  the  temporal.  The  other  bones  of  the  face  are 
two  little  plates  called  lachrymals,  grooved  for  the  nasal 
ducts,  the  passages  by  which  the  tears  are  carried  from  the 
eyes  into  the  nose  ;  and  the  inferior  turbinated  bones,  a  pair 
of  thin  curved  laminae  which  project  into  the  nasal  fossae. 

20.  Peculiarities  of  the  Human  Skeleton. — The  most 
remarkable  peculiarities  of  the  skeleton  of  man,  as  compared 
with  other  animals,  are  connected  with  the  maintenance  of 
the  erect  posture. 

The  foot  has  a  broad  sole,  and  is  arched  both  from  behind 
forwards,  and  also  from  side  to  side,  so  as  to  give  elasticity 
to  the  step. 

The  straight  position  of  the  knee  is  characteristically 
human,  no  other  animal  but  man  being  supported  on  ex 
tended  knee  joints;  for  though  birds  are  also  bipeds,  thej 
have  the  knees  flexed  in  standing.  The  human  knee  joint 
is  so  constructed  that,  when  fully  extended,  it  remains  in 
that  position  without  muscular  exertion,  so  long  as  the 
weight  of  the  body  presses  clown  on  it.  And  this  can  easily 
be  demonstrated;  for  the  patella  is  situated  in  the  tendon  of 
the  extensor  nmscle  of  the  knee,  and  when  it  is  loose,  that 
muscle  is  evidently  relaxed :  now,  when  one  stands  with  the 
knees  straight,  the  patella  can  be  felt  with  the  hand  to  be 
hanging  perfectly  slack;  but  as  soon  as  the  foot  is  lifted 


38  ANIMAL  PHYSIOLOGY. 

from  tlie  ground  it  becomes  tightened,  because  then  it  is 
only  by  muscular  effort  that  the  knee  is  kept  straight. 

The  femur  in  man  is  longer  than  in  other  animals;  and  by 
the  length  of  this  bone,  when  we  stoop  with  bent  knees  and 
resting  on  the  balls  of  the  toes,  the  pelvis  is  thrown  suffi- 
ciently backwards  to  balance  the  bending  of  the  body 
forwards  (fig.  27).  If  the  thigh  bones  were  short,  it  would 
be  difficult  to  pick  an  object  off  the  ground. 

The  pelvis  also  is  short,  expanded,  and  strong :  the  pillars  of 
bone  which  convey  the  weight  from  the  sacrum  to  the  thigh- 
bones are  stouter  than  the  corresponding  parts  which  have 
110  such  function  in  other  animals;  and  the  expanded  blades 
of  the  iliac  bones  both  give  surface  for  the  attachment  of 
the  large  glutei  muscles  by  which  the  trunk  is  extended  on 
the  top  of  the  thigh-bones,  and  also  help  to  support  the 
viscera  above  them. 

The  bodies  of  the  vertebrse  increase  rapidly  in  size  from 
the  cervical  to  the  last  lumbar,  so  as  to  bear  the  accumulated 
weight  which  they  support;  and  the  transverse  processes  of 
the  thoracic  and  lumbar  regions  are  thrown  remarkably  back 
on  the  arch,  so  as  to  bring  the  bodies  as  much  as  possiblo 
forward  into  the  visceral  cavity;  a  circumstance  which  will 
at  once  strike  any  one  who  compares  even  in  a  cursory 
fashion  the  lumbar  vertebrse  of  a  rabbit,  sheep,  or  ox,  with 
those  of  the  human  subject.  In  the  thoracic  region,  the 
ribs,  with  the  exception  of  the  two  last  pairs,  being  attached 
by  distinct  articulations  to  the  sides  of  the  bodies  of  the 
vertebrse  and  to  the  transverse  processes  as  well,  have  a 
direction  backwards  as  well  as  outwards  given  to  them,  by 
which,  before  arching  forwards,  they  include  in  their  circuit 
two  great  fossse  at  the  sides  of  the  column,  which  contain  a 
large  part  of  the  lungs.  -  .- 

Even  the  peculiarities  of  the  human  skull  are  closely  con- 
nected with  the  adaptation  to  the  erect  posture.  It  has 
already  been  pointed  out  that  in  quadrupeds  the  head  is 
suspended  by  a  strong  elastic  ligamentum  nuchse;  and  it  is, 
in  addition,  supported  by  muscular  action;  but,  in  man,  the 
head  is  balanced  on  the  top  of  the  atlas  when  he  stands 
erect.  This  is  an  arrangement  altogether  peculiar  to  man, 
and  is  accomplished,  in  the  first  place,  by  the  bones  of  the 


PECULIARITIES   OF   THE   HUMAN   SKELETON.  39 

face  being  comparatively  light,  and,  secondly,  by  changes  in 
the  form  of  the  cranium,  connected  with  the  large  size  of  the 
brain.  These  changes  consist  mainly  in  the  base  of  the 
skull  in  front  of  the  foramen  magnum,  being  shortened  and 
curved  downwards,  and  in  the  roof  being  greatly  elongated 
and  arched,  so  that  the  part  of  the  occipital  bone  behind 
the  foramen  magnum,  which  in  a  quadruped  looks  back- 
wards, is  turned  downwards,  and  a  large  part  of  the  brain  is 
thus  made  to  lie  further  back  than  the  condyles  by  which 
the  occipital  bone  articulates  with  the  atlas. 


Fig.  18. — LlG  AMENTUM  NlJCILE  OF  THE  HORSE. 

The  elongation  of  the  face  downwards  may  be  mentioned 
as  a  human  peculiarity,  as  well  as  the  want  of  projection 
forwards.  This  elongation  is  partly  in  connection  with  the 
development  of  spaces,  in  which  the  voice  reverberates  and 
acquires  resonance,  but  cannot  be  altogether  accounted 
for  by  that  consideration.  Rather,  it  is  a  physiognomical 
peculiarity  of  man,  like  the  presence  of  a  chin,  well  developed 
in  the  higher  varieties  of  the  race,  but  not  in  subservience 
to  any  special  function. 

In  the  skeleton  of  the  upper  limb  there  is  no  mechanism 
altogether  peculiar  to  man ;  the  completely  developed 
clavicle,  freely  moving  shoulder  joint,  pronation  and  supin- 
ation  of  the  forearm,  opposability  of  the  thumb.,  and.  Qom- 


40  AtflMAt   PHYSIOLOGY. 

plete  power  of  handling  and  fingering,  are  all  found  among 
the  lower  animals. 

21.  Skeletal  Textures. — The  textures  found  in  the  skeleton 
are  bone,  cartilage,  and  the  fibrous  tissues.  The  fibrous 
tissues  have  been  already  considered.  Bone,  which  is  the 
prevalent  texture  in  the  adult  skeleton,  makes  its  first  ap- 
pearance always  either  in  cartilage  or  fibrous  tissue,  more 
frequently  in  cartilage ;  therefore,  it  is  convenient  to  explain 
the  nature  of  cartilage  first. 

Cartilage,  in  its  most  frequent  form  distinguished  as  true  or 
hyaline  cartilage,  is  a  firm  texture  capable  of  a  marked  amount 
of  flexion,  but  breaking  with  a  smooth  fracture  when  it  is  sought 
to  bend  it  further  than  its  flexibility  will  allow.  It  presents, 
tinder  the  microscope,  a  clear  or  slightly  granular  matrix, 
with  nucleated  corpuscles  imbedded  in  it,  of  variable  size,  and 
lodged,  singly  or  in  groups,  in  hollows,  which  are  either  of  a 
rounded  form,  or  with  flattened  sides  and  rounded  angles, 
and  are  never  branched.  The  limits  of  these  hollows  are 
denser  than  the  surrounding  matrix,  and  are  termed  capsules 
of  the  corpuscles.  Cartilage  is  completely  devoid  of  blood- 
vessels, for  though  occasional  vessels  occur  in  large  masses, 
as,  for  example,  in  the  costal  cartilages,  they  are  always  lodged 
in  canals  along  with  a  small  amount  of  connective  tissue. 
But  the  matrix  is  freely  permeated  by  nourishment  from  the 
vessels  round  about;  for  cartilage  is  capable  of  rapid  growth, 
and  its  growth  is  marked  by  changes  throughout  its  substance. 
In  growing  cartilage,  the  corpuscles  are  seen  in  groups  in 
every  stage  of  multiplication.  One  will  be  found  with  two 
or  more  nuclei;  another  partially  divided  into  two  or  more 
parts,  with  septa  springing  up  between  them;  while,  in 
other  instances,  the  septa  are  completed,  and  exhibit  various 
thicknesses  of  matrix  between  them,  towards  which  the 
divided  corpuscles  still  present  flattened  sides.  The  matrix 
of  cartilage  is  converted  by  prolonged  boiling,  not  into  gela- 
tin, but  into  chondrin,  a  closely  allied  substance,  which,  like 
gelatin,  dissolves  in  hot  water,  and  forms  a  jelly  on  cooling ; 
but  which  differs  slightly  in  composition,  and  has  some 
distinctive  chemical  reactions.  This  is  the  more  remarkable, 
as  bone  yields  by  boiling,  not  chondrin,  but  gelatin. 

The  coating  of  cartilage  on  surfaces  of  bone  which  glide 


CARTILAGE. 


41 


one  on  another,  is  termed  articular  cartilage,  and  has  the 
peculiarity  that,  towards  the  surface,  the  corpuscles  are  in 
groups  flattened  parallel  to  the  surface,  while  in  the  deep 
parts  they  are  in  vertical  groups. 


Fig.  19.—  COSTAL  CARTILAGE. 


Fig.  21.— PvETicTOAE  CAIITIL-         Fig.  20.— ARTICTOAE  CARTILAGE. 
AGE,  from  the  Epiglottis.  Vertical  Section. 

A  peculiar  variety  of  cartilage,  called  yellow  or  reticular, 
which  occurs  in  the  epiglottis  and  a  few  other  places,  depends 
on  the  matrix  being  pervaded  with  a  densely  felted  substance 
similar  to  elastic  fibrous  tissue,  but  more  brittle. 


42  ANIMAL  PHYSIOLOGY. 

Plates  of  matted  fasciculi  of  fibrous  tissue,  with  elements 
of  cartilage  sometimes  interspersed,  occur  in  various  positions, 
but  particularly  as  movable  discs  between  the  articular  sur- 
faces of  certain  joints,  such  as  the  knee,  and  combine  the 
elastic  resistance  of  cartilage  with  toughness  which  endures 
the  action  of  rubbing,  and  are  named  jibro-cartilages. 

22.  Bone  is  a  more  complex  tissue  than  cartilage;  its 
complexity  depending  011  the  impermeability  of  its  matrix  to 
fluids,  and  the  consequent  necessity  of  canals  for  nutrition. 
The  matrix  consists  two-thirds  of  mineral  matter,  principally 
phosphate  of  lime  with  some  carbonate  of  lime,  and  the 
remaining  third  of  animal  matter;  the  two  being  so  intimately 
blended  that  they  form  a  homogeneous  mass,  translucent  in 
thin  sections.  When  the  animal  matter  is  removed  by  cal- 
cination the  form  of  the  bone  still  remains ;  and  when  the 
mineral  matter  has  been  gradually  dissolved  by  dilute  hydro- 
chloric acid,  the  animal  matter  retains  the  same  bulk  and 
microscopic  structure  as  before,  presenting  the  consist- 
ence and  flexibility  of  cartilage,  but  yielding  gelatin  by 
boiling. 

The  only  microscopic  structures  common  to  all  bone  are  the 
bone-corpuscles,  which  are  nucleated  corpuscles  characterized 
by  a  multitude  of  fine  processes,  and  are  imbedded  in  hollows 
of  corresponding  shape  and  size,  called  lacunce;  while  their 
processes  occupy  exceedingly  fine  canaliculi,  which  radiate 
from  the  lacunae,  those  of  one  lacuna  inosculating  with  those 
of  others,  so  that  fluids  may  be  conveyed  from  one  lacuna  to 
another. 

Bony  tissue  is  found,  however,  in  two  different  forms,  the 
cancellated  and  the  compact.  Cancellated  or  spongy  tissue, 
such  as  one  finds  in  the  bodies  of  the  vertebrae,  the  tarsal 
bones,  and  the  ends  of  long  bones,  consists  of  minute  spicules 
and  occasional  laminae  of  bone  with  the  spaces  or  meshes 
between  them  filled  with  fine  connective  tissue,  copiously 
supplied  with  blood-vessels,  and  loaded  more  or  less  with 
adipose  matter.  Compact  or  solid  bony  tissue,  such  as  is 
found  in  the  shafts  of  the  long  bones,  is  traversed  by  blood- 
vessels, and  presents  a  remarkable  microscopic  arrangement 
connected  therewith.  The  passages  for  the  blood-vessels, 
named  Haversian  canals,  after  Havers,  who  first  mentioned 


BONE. 


them,  vary  from  y-^j-  to  -% J^  of  an  inch  in  diameter ;  they 
enter  from  the  surface  of  the  bone  by  multitudes  of  minute 
oblique  openings  visible  with  the  naked  eye,  and  run  for  the 
most  part  longitudinally,  connected  however  by  numerous 
short  canals,  which  have  a  more  transverse  direction.  The 


Fig.  22. — LACUNAE  AND         Fig.  23. — TRANSVERSE  SECTION  OB= 
CANALICULI.  COMPACT  OSSEOUS   TISSUE,      a, 

Haversian  canal ;  &,  Lacunce  in 
concentric  rings. 

tissue  is  arranged  in  concentric  laminae  around  the  Haversian 
canals,  so  that  circles  of  lacunae  are  seen  surrounding  the 
transverse  sections  of  the  canals,  and  such  an  arrangement  of 
concentric  rings  is  called  an  Haversian  system.  The  whole 
compact  tissue  is  made  up  of  such  systems,  the  interstices 
being  filled  with  fragments  of  similar  laminae  which  were 
formerly  complete,  but  of  which  the  other  portions  have 
been  absorbed  so  as  to  leave  gaps  or  absorption-spaces,  sub- 
sequently filled  up  by  new  systems  developed  concentrically 
from  the  circumference  inwards,  till  they  have  closely  grasped 
the  blood-vessels  in  the  centre. 

The  arteries  for  the  supply  of  bone  subdivide  in  the 
fibrous  membrane  by  which  each  bone  is  surrounded,  the 
periosteum,  and  from  this  membrane  small  branches  pass  all 
over  the  surface  into  the  openings  of  the  Haversian  canals. 
The  veins  emerge  by  comparatively  few  orifices  of  larger  size, 


44 


ANIMAL  PHYSIOLOGY. 


which.,  in  long  bones,  are  found  near  the  articular  extremities. 
The  marrow  cavities  in  the  shafts  of  long  bones  may  be 
looked  on  as  of  the  same  description  as  the  cancellations  in 
the  spongy  tissue  at  the  extremities  of  the  same  bones,  with 
which  they  communicate.  The  marrow  is  vascular  connec- 
tive tissue  of  a  delicate  descrip- 
tion, loaded  with  adipose  cells,  and 
has  usually  a  special  artery,  the 
BO -called  nutrient  artery,  which 
pierces  the  bone,  and  supplies  both 
the  marrow  and  the  innermost  part 
of  the  osseous  tissue. 

23.  Bone  is  formed,  as  has  been 
ctated,   either    from    cartilage    or 
fibrous  tissue.     All  bones  of  con- 
siderable thickness  are   originally 
cartilaginous,  and  their  ossification 
begins  in  the  centre  of  the  mass. 
The  first  step  preliminary  to  this 
process  of  ossification  is  the  multi- 
plication of  vessels  within  cana]s, 
and  the  absorption  before  them  of 
a  certain  amount  of  cartilaginous 
matrix.     When  a  section  is  made 
Fig.   24.  —  Vertical   Section  through  the  plane  of  contact  of  a 
through  the  plane  of  ossifi-  centre  of  ossification  and  the  sur- 
cation  at  the  upper  end  of  rounding  cartilage,  the   cartilage- 
•    S^^ht^tolS -rpuscies   are   seen  arranged  in 
in  vertical   columns,    and  rows     placed  ^  vertically     to     the 
altered  matrix  between ;  b,  plane     of    ossification  j     and    be- 
granular  deposit  spreading  tween     these     rows     there     pro- 
inspicules;c,  true  bone.      ^     ^to     the    ^  matrix      opaque 
spicules,  which  consist  of  granules  of  calcified  matter,  dis- 
tinct  one   from   another,    and   reflecting   the  light.      By  a 
further  deposition  of  granules,  the  cartilage-corp'uscles  become 
hid  from  view  and  closely  surrounded;  and  in  some  instances 
mineral  deposit  takes  place  also  within  the  capsules.     By 
still  further  deposition  of  mineral  matter,  the  matrix  becomes 
homogeneous  and  transparent,  and  within  the  ossifying  border 
spaces  are  formed  by  absorption.     Within  these  spaces  there 


BONE.  45 

is  a  free  growth  of  corpuscles  (termed  osteoblastic)  and  blood- 
vessels. Whether  the  corpuscles  are  derived  from  those  of 
the  cartilage,  or  from  the  connective  tissue  round  the  vessels, 
is  not  settled ;  but  they  become  imbedded  in  a  new  deposi- 
tion of  calcified  matrix,  which  leaves  them,  with  freely  in- 
tercommunicating branches,  and  are  thus  converted  into  true 
bone-corpuscles. 

The  first  deposit  of  bone  is  dense  and  irregular  :  if  the 
spaces  formed  in  this  by  absorption  accumulate,  cancellated 
tissue  is  the  result ;  but  if  they  become  filled  with  a  new 
deposit  of  bone,  this  deposit  takes  place  in  concentric  rings, 
gradually  closing  round  the  blood-vessels,  and  compact  tissue 
is  produced. 

In  cancellated,  as  well  as  in  compact  tissue,  there  is  con- 
tinual deposit  and  reabsorption  of  bone;  but  in  the  compact 
tissue,  the  osseous  substance  is  in  such  proportion  to  the  vas- 
cular, as  to  surround  the  vessels;  while  in  the  cancellated, 
the  vascular  connective  tissue,  or  red  marrow,  is  in  such 
quantity  as  to  surround  the  osseous  spicules. 

When  bone  is  developed  from  fibrous  tissue,  there  is  no 
granular  stage  in  ossification;  but  bony  tissue  is  laid  down  at 
once,  as  in  spaces  formed  by  absorption. 

24.  While  cartilage  is  capable  of  rapid  growth,  as  has  been 
already  stated,  by  multiplication  of  its  corpuscles  and  expan- 
sion of  its  matrix,  bony  tissue  is  capable  of  very  little  ex- 
pansion, and  increases  in  bulk  by  addition  to  its  surfaces, 
where  it  is  in  contact  with  cartilage  or  fibrous  tissue.  Thus, 
a  ring  of  silver  fastened  round  the  wing-bone  of  a  young 
pigeon  becomes  gradually  imbedded  and  covered  in  by  the 
new  depositions  of  bone  on  the  surface,  while,  by  the  absorp- 
tion which  is  at  the  same  time  going  on  internally,  it  may 
even  come  to  lie  in  the  enlarged  cavity  of  the  bone.  John 
Hunter  found,  that  if  two  holes  were  bored  in  a  bone  of  a 
young  animal,  at  a  measured  distance  one  from  the  other, 
afterwards  when  the  bone  had  grown  longer,  the  holes  remained 
separated  by  the  same  interval  as  at  first.  Even  apart,  how- 
ever, from  the  circumstance  that  subsequent  observers  on 
repeating  Hunter's  experiment  have  obtained  a  different 
result,  it  must  be  admitted  that  osseous  tissue  has  some 
power,  although  limited,  of  interstitial  expansion,  seeing  that 


46  ANIMAL   PHYSIOLOGY. 

the  body  of  tlie  lower  jaw  elongates  by  that  means  sufficiently 
to  make  room  within  it  for  the  permanent 
teeth,  and  an  expansion  of  the  same  sort 
takes  place  in  the  frontal  bone  between  the 
frontal  eminences  and  the  margins  of  the 
orbits.  But  the  principal  elongation  of  the 
bones  of  the  limbs  is  provided  for  by  the 
extremities  of  the  bones  being  furnished 
with  separate  epiphyses  or  supplementary 
centres  of  ossification,  between  which  and 
the  shaft  there  remains,  as  long  as  the  growth 
of  the  bone  continues,  a  thin  plate  of  carti- 
lage, which  is  as  rapidly  growing  intersti- 
tially  as  it  is  converted  into  bone  at  its 
surface.  So,  also,  the  bones  of  the  skull 
expand  principally  by  additions  at  their 
edges,  and  when  premature  obliteration 
(synostosis)  of  any  suture  occurs,  it  causes 

•p-     25 TIBIA  at  arres^   °^   growth   at  right   angles   to   the 

18  years  of  age,  suture,  compensated  for  by  additional  growth 
showing  the  su-  in  other  directions,  producing  a  variety  of 
perior  and  in-  anomalous  forms  of  skull. 

fenorepiphyses.  25.  The  Joints  or  Articulations,  by 
which  the  different  bones  are  joined  together,  may  be 
divided  primarily  into  movable  and  immovable. 

All  the  bones  of  the  skull,  with  the  exception  of  the  lower 
jaw,  are  united  together  by  immovable  articulations  or  sutures, 
many  of  them  rendered  firmer  by  the  doyetailing  of  compli- 
cated serrations  of  the  articulating  edges.  Such  articulations 
are  serviceable  for  purposes  of  growth,  as  has  been  already 
explained. 

Movable  articulations  are  divisible  into  complete  or  per- 
fect, and  incomplete  or  imperfect. 

Incomplete  joints  are  those  in  which  the  opposed  sur- 
faces of  bone  are  united  by  intervening  substance  of  a 
yielding  description ;  and  the  most  notable  example  of  this 
mode  of  union  is  found  in  the  vertebral  column.  The  arches 
of  the  vertebrae  are  united  by  pairs  of  complete  joints,  but 
their  bodies,  the  parts  through  which  the  weight  of  the  trunk 
is  principally  conducted,  are  joined  by  intervertebral  discs, 


JOINTS   OR   AKTIC0IAT10NS. 


47 


consisting  exteriorly  of  rings  of  oblique  fibres,  and  more 
deeply  of  pulpy  tissue,  rich  in  corpuscles  like  cartilage- 
corpuscles.  These  discs  are  so  many  elastic  pads  which  are 
useful  in  preventing  shocks,  and  while  little  movement  is 
allowed  by  them  between  each  pair  of  bones,  the  amount 
permitted  in  the  whole  column  is  very  considerable. 


Fig.  26. — LUMBAR  INTERVEIITEBRAL  Disc.  A,  front  view.  B,  hori- 
zontal section.  C,  section  from  before  backwards,  a,  fibrous 
rings;  6,  pulpy  tissue;  c,  articular  surfaces  in  contact;  d,  anterior 
common  ligament;  e,  posterior  common  ligament. 

Complete,  joints  are  those  in  which  the  surfaces  of  bones 
coated  with  articular  cartilage  glide  one  against  the  other, 
while  the  ligaments  which  bind  them  together  are  placed 
round  about.  Internal  to  the  ligaments  there  extends  from 
the  circumference  of  one  articular  surface  to  that  of  the 
other,  a  delicate  synovial  membrane,  so  called  because  from 
it  there  exudes  sufficient  fluid,  termed  synovia,  to  moisten 
the  surfaces  of  the  shut  cavity  which  it  encloses. 

In  the  complete  joints,  the  movements  allowed  vary  greatly 
both  in  character  and  degree.  In.  many,  as,  for  example, 
between,  the  individual  bones  constituting  the  second  range  of 
the  carpus,  and  between  some  of  the  tarsal  bones,  the  move- 
ment is  extremely  slight  (arthrodia),  and  the  principal  advan- 
tage gained  appears  to  be  elastic  resistance  to  pressure,  by  the 
weight  being  thrown  upon  tense  ligaments.  Others,  in  which 
the  movements  are  extensive,  may  be  rudely  compared  with 
joints  made  by  mechanicians.  Thus,  the  shoulder  and  the 
hip  maybe  termed  ball  and  socket  joints,  there  being  in  each 
a  rounded  surface  which  moves  in  all  directions  against  a 


48  ANIMAL   PHYSIOLOGY. 

cup;  the  elbow  and  the  ankle  furnish,  examples  of  hinge- 
joints,  permitting  angular  movement  in  only  one  path,  that 
of  flexion  and  extension;  and  in  the  articulation  of  the  first 
and  second  vertebrae,  we  have  an  instance  of  a  pivot-joint, 
the  odontoid  process  of  the  axis  or  second  vertebra,  being  the 
pivot  round  which  the  atlas  or  first  vertebra  revolves. 

26.  Mechanics  of  the  Skeleton. — By  the  contraction  of 
muscles  passing  over  joints  and  attached  to  bones,  the  parts 
of  the  skeleton  are  thrown  into  different  positions,  so  that 
we  are  enabled  to  support  ourselves  in  different  attitudes  and 
to  move  about. 

For  the  support  of  the  body,  it  is  first  of  all.necessary 
that  the  centre  of  gravity  be  within  the  basis  of  support, 
whether  that  consist  of  one  foot  or  of  both;  and  to  accom- 
plish this  the  body  is  instinctively  balanced  by  compensatory 
deviations  of  its  different  parts  from  the  vertical  position. 
The  weight  of  the  body  in  standing  falls  on  the  arch  of  the 
instep,  the  piers  of  which  are  the  heel  and  the  balls  of  the 
toes.  When  the  feet  are  together  and  the  knees  straight, 
both  the  tibia  and  the  femur  are  thrown  forwards  further  at 
their  upper  ends  than  their  lower,  and  over-extension  of  the 
knee  is  prevented  by  the  construction  of  that  joint;  while 
the  weight  transmitted  from  the  vertebral  column  is 
received  by  the  haunch  bones  at  a  point  further  back 
than  where  these  bones  articulate  by  the  hip-joints  with 
the  thigh  bones.  The  leg  is  prevented  from  falling  for- 
wards over  the  foot  at  the  ankle  joint  by  the  action  of  the 
muscles  of  the  calf,  the  soleus  and  gastrocnemius  ;  and  this 
is  probably  the  only  instance,  in  the  standing  posture,  of  a 
considerable  weight  being  permanently  supported  by  muscular 
power.  At  the  hip-joint  the  weight,  being  behind,  makes 
tense  the  ligaments  in  front;  and  the  muscles  passing  over 
the  back  of  that  joint,  while  they  may  be  felt  to  be  rigid 
during  their  activity  in  recovering  the  body  from  stooping, 
are  flaccid  when  the  erect  position  is  attained.  The  vertebral 
column  is  balanced  by  being  curved  in  different  directions : 
in  the  loins  it  is  thrown  forwards;  where  it  supports  the 
chest  it  is  curved  well  backwards;  it  turns  forwards  again  in 
the  upper  part  of  the  chest  and  at  the  root  of  the  neck;  and 
on  the  top  of  the  column  the  head  maintains  its  position  by 


SUPPOHT   OF   THE   BODY. 


49 


balance.  Thus  it  will  be  observed  that  in  preserving  the 
erect  posture,  the  muscles  principally  act  in  steadying  the 
body,  but  have  little  of  its  weight  thrown  on  them;  and  this 
is  exceedingly  important,  as  muscular  contraction  is  a  vital 
action  involving  expenditure  of  force,  and  very  exhausting. 


Fig.  27. — Illustrates  the  preservation  of  the  centre  of  gravity 
within  the  basis  of  support.      . 

The  body  is  supported  with  still  less  muscular  effort  in 
the  position  called  standing  at  ease  than  at  attention.  In 
standing  at  ease  the  weight  is  borne  principally  on  one  foot, 
while  the  other  assists  lightly  as  a  prop.  The  limb  on  which 
support  is  made  has  the  knee  straight,  and  is  inclined  above 
towards  its  own  side,  so  as  to  bring  the  weight  of  the  trunk 
over  the  foot;  the  haunch  of  the  other  side  is  allowed  to 
drop  to  a  lower  level  than  its  fellow,  sending  the  lower  end 
of  the  vertebral  column  into  an  oblique  position;  and  the 
trunk  is  kept  vertical  by  the  column  being  thrown  into  a 
spiral  curve. 

When  a  heavy  weight  is  carried  in  front  of  the  body, 
the  trunk  is  thrown  sufficiently  back  to  bring  the  centre 
of  gravity  of  the  whole  mass  within  the  basis  of  support ; 
for  this  reason  portly  persons,  in  whom  the  weight  of 
the  abdominal  region  is  greatly  increased,  hold  themselves 
H  D 


50  ANIMAL   PHYSIOLOGY. 

particularly  erect.  When  the  upper  part  of  the  "body  is  Lent 
forward,  the  lower  part  is  carried  backwards;  and  if  the 
knees  be  bent,  the  projection  forwards  of  the  legs  is  balanced 
by  the  bending  backwards  of  the  thigh.  When  the  knees 
are  completely  bent,  the  heel  is  raised  from  the  ground, 
because  the  joints  at  the  balls  of  the  toes  are  required  to 
supplement  the  ankle  in  bending  the  leg  sufficiently  down 
to  bring  the  centre  of  gravity  forwards  over  the  base  of  sup- 
port; and  that  is  the  reason  why,  in  such  a  position,  the 
heel  is  more  raised  when  the  trunk  is  upright  than  when  it 
is  stooped  forwards. 

27.  In  walking  and  running  the  weight  of  the  body  is 
thrown  from  one  limb  to  the  other  alternately,  and,  except  in 
exceedingly  slow  marching,  balance  is  not  yet  completely 
established  on  one  limb  when  the  weight  is  shifted  to  the 
other.  Before  the  limb  which  is  advanced  reaches  the  ground, 
the  body  is  propelled  forwards  by  the  straightening  of  the 
ankle  of  the  foot  which  is  behind;  it  is  then  pulled  over  on 
the  hip  of  the  advanced  limb  by  the  muscles  on  the  outside 
of  that  joint  (glutens  medius  and  minimus),  and,  especially 
in  long  steps,  it  is  drawn  forwards  to  the  advanced  limb  by 
a  strong  mass  of  muscles  extending  between  the  pelvis  and 
inside  of  the  thigh  (the  adductors). 

The  difference  between  walking  and  running  is  two- 
fold. First,  in  walking  the  heel  is  brought  to  the  ground, 
while  in  running  it  is  not;  yet  the  foot,  at  the  moment 
when  it  is  used  to  propel  the  body  in  running,  is  as  ad- 
vantageously placed  as  in  walking ;  for  the  leg  is  so  much 
sloped  forwards  that  the  angle  between  it  and  the  foot  is 
quite  as  sharp  as  it  ever  is  in  walking.  Secondly,  whereas 
in  walking  the  whole  propulsive  action  is  from  the  foot,  in 
running,  the  knee  and  hip-joints  being  both  greatly  bent,  a 
vast  additional  impulse  is  given  by  their  simultaneous  exten- 
sion. So  also  in  leaping,  all  the  joints  of  thelower  limb 
are  flexed  in  preparation  for  the  leap,  and  the  impetus  given 
by  their  sudden  extension  propels  the  body  through  the  air. 
But  leaping  differs  from  running,  in  that  the  limbs  are 
extended  together  instead  of  acting  alternately. 

28.  In  the  movements  of  the  skeleton,  all  the  three  orders 
of  levers  are  employed.    In  extending  the  fore-arm,  as  in  box- 


WALKING  AND   RUNNING. 


51 


ing,  a  lever  of  the  first  order  is  illustrated ;  tlie  hand  being 
the  weight,  the  extensor  of  the  elbow  the  power,  and  that 
joint  the  fulcrum  placed  between  the  weight  and  power. 
But  when  the  elbow  is  straightened  in  raising  the  body  on 
the  hands,  then  the  superincumbent  weight  falls  at  the 
elbow,  between  the  extensor  muscle,  which  is  still  the  power, 
and  the  hand,  which  is  now  the  fulcrum;  and  the  second  order 
of  levers  is  illustrated.  When  a  weight  held  in  the  hand  is 
raised  by  bending  the  elbow,  the  flexor  muscles  in  front  of 
the  joint  are  those  which  act;  and,  as  they  are  situated 
between  the  fulcrum  and  weight,  a  lever  of  the  third  order 
is  brought  into  action. 


Fig.  28. — THE  THREE  ORDERS  OF  LEVERS,  illustrated  at  the  Elbow. 
The  muscles  of  the  calf  (the  gastrocnemius  and  soleus), 
passing  down  to  the  tenclo  Achillis,  are  concerned  in 
actions  illustrating  all  three  kinds  of  lever.  "When  the 
foot  is  raised  and  the  toes  depressed,  as  in  working  a 
pedal,  the  weight  is  at  the  toes,  and  the  ankle-joint  is  the 
fulcrum  of  a  lever  of  the  first  order ;  when  we  rise  on  tip- 
toe it  is  the  muscles  of  the  calf  which  raise  the  heel,  the 
fulcrum  is  at  the  toes,  and  the  weight  of  the  body  falls  on 
the  ankle  after  the  fashion  of  a  lever  of  the  second  order; 
and,  lastly,  in  the  slighter  action  of  the  same  muscles,  when 
the  heel  is  kept  to  the  ground  by  the  weight  of  the  body,  the 
force  which  prevents  the  body  falling  forwards  is  applied 
at  the  upper  attachments,  while  the  ankle  is  the  fulcrum, 


52  ANIMAL  PHYSIOLOGY. 

.and  the  forward  inclination  of  the  body  above  is  the  resist- 
ance, so  that  the  mechanism  is  that  of  a  lever  of  the  third 
order. 


Fig.  29. — THE  THREE  ORDERS  OF  LEVERS,  illustrated  in  the  Foot. 


CHAPTER  IV. 
MUSCLES. 

29.  THE  active  element  in  which  the  force  resides  by  which 
not  only  the  bones  and  joints  are  set  in  motion,  but  likewise 
all  the  movements  of  the  organs  are  accomplished,  is  called 
muscular  fibre. 

Muscular  fibre  presents  two  great  varieties,  the  striped 
and  the  unstriped.  The  striped  is  the  more  complex,  and,  as 
it  is  the  variety  of  which  all  the  muscles  consist,  and  over  which 
the  will  has  control,  it  is  likewise  termed  voluntary  muscular 
fibre ;  while  the  unstriped  or  smooth  variety,  being  employed 
in  the  viscera,  blood-vessels,  and  other  structures  subserving 
the  purposes  of  organic  life,  and  beyond  the  control  of  the 
will,  is  termed  involuntary  or  organic  muscular  fibre.  There 
are,  however,  various  instances  in  which  striped  muscular 
fibre  is  not  under  the  direction  of  the  will,  the  principal  being 
the  heart ;  and  there  is  a  circumstance  other  than  the  rela- 
tion to  the  will,  on  which  more  probably  the  kind  of  fibre 
used  in  each  structure  depends,  namely,  that  striped  muscle  con- 
tracts suddenly  when  irritated,  and  becomes  suddenly  relaxed, 
whereas  the  unstriped  contracts  slowly  and  is  relaxed  slowly. 
The  heart  is  required  to  contract  rapidly,  and  striped  muscu- 
lar fibre  is  used  in  its  construction.  The  mode  of  contrac- 
tion of  striped  muscle  is  exemplified  by  the  immediate 
response  of  the  muscles  of  the  skeleton  to  the  impulses  of 
the  will ;  while  the  contraction  of  unstriped  fibre  may  be 
seen  by  watching  the  enlargement  and  diminution  of  the 
pupil,  occasioned  by  altering  the  focus  of  the  eye  or  the 
amount  of  light  admitted  to  it  (p.  242).  But  birds,  which 
probably  require  more  rapid  action  of  the  iris  in  connection 
with  their  extraordinary  keenness  of  vision,  have  the  muscu- 
lar tissue  of  the  iris  composed  of  striped  fibres;  and  in  niam- 


ANIMAL   PHYSIOLOGY. 


mals  the  muscles  of  the  tympanum,  which  are  stimulated 
precisely  in  the  same  way  as  the  iris,  are  of  the  striped 
description. 


a,    '  ~b    ' 

Pig.  30. — STRIPED  MUSCULAR  FIBRE,  a,  undisturbed  fibre ; 
treated  with  acetic  acid,  to  show  the  nuclei ;  c,  the  striated 
substance  torn  and  the  sarcolemma  uninjured;  d,  fibre  teased 
out  to  show  the  fibrillse  ;  c,  fibre  broken  into  discs  above,  a*nd 
showing  fibrillse  below  ;  /,  termination  of  nerve,  after  Kiihne. 

30.  Striped  Muscular  Tissue  consists  of  very  long  fibres 
which,  in  their  best  developed  varieties,  approach  •—-$ 
of  an  inch  in  diameter.  In  most  instances  each  fibre 
exhibits  a  delicate  sheath  or  sarcolemma,  which,  in  its 
resistance  of  reagents,  resembles  elastic  tissue.  The  sar- 
colemma is  filled  with  substance  which  is  closely  striped 
or  striated  transversely,  the  striation  depending  on  a 
regular  alternation  of  parts  of  different  refractive  pro- 
perties. By  careful  manipulation  this  striated  substance 
may  be  separated  up  into  a  bundle  of  fine  threads  called 
jfibrillce,  each  of  which  exhibits  the  same  alternation  of  parts 
which  causes  the  striation  of  the  fibre,  and  may  be  looked  on 
as  a  series  of  dark  or  highly  refractive  parts,  the  sarcous 
elements  of  Bowman,  connected  by  means  of  a  less  dense 
material.  By  other  modes  of  manipulation  the  striped  fibres 
may  be  partially  cleft  across,  so  as  to  present  the  appearance 
of  a  series  of  discs ;  and  there  is  no  sufficient  reason  for  sup- 
posing that  the  division  into  fibrillse  is  more  natural  than 
this  division  into  discs.  To  the  interior  of  the  sarcolemma 
are  adherent  a  number  of  elongated  nuclei ;  and  it  is  inipor- 


tlNSTRIPEt)    MUSCULAR   TISSUE.  65 

taut  to  observe  that  in  the  frog,  and  also  in  early  foetal 
development  of  mammals,  the  nuclei  are  imbedded  in  the 
middle  of  the  striated  contractile  substance.  The  fibres  of 
the  heart  have  no  sarcolemma. 

31.  Unstriped  Muscular  Tissue  is  often  arranged  in 
bands  of  indefinite  length  like  the  striped  fibres ;  but  even 
when  this  is  the  case,  it  consists  of  a  series  of  elongated  fusi- 
form corpuscles  varying  usually  from  ^o"  to  ^^  of  an  inch 
in  length,  and  known  as  fibre-cells,  although  possessing  no 
proper  cell-wall.  These  fibre-cells  are  flat,  with  long  taper- 
ing extremities,  an  elliptic  or  rod-like  nucleus  in  the  middle, 
and  at  each  end  of  the  nucleus  usually  a  few  granules.  They 
are  of  dense  consistence,  and  adhere  one  to  another  tena- 
ciously by  means  of  a  material  acted  on  by  nitric  acid. 


Fig.  31.— FIBRE-CELLS  OF  UNSTRIPED  FIBRE. 

32.  New  muscular  fibres  of  both  the  striped  and  unstriped 
variety,  would  appear  to  be  capable  of  being  developed  from 
connective-tissue-corpuscles.      In  both  varieties  the  connec- 
tion with  the  nervous  system  is   effected  by  filaments    of 
nerves  entering  the  interior  of  the  fibre ;  in  striped  fibres  the 
filaments  end  in  swellings  or  expansions  in  or  on  the  striated 
substance,  and    in  unstriped  fibres  certain  observers  trace 
them  to  the  nuclei  of  the  fibre-cells. 

33.  Muscular  substance  consists  in  greater  part  of  muscle* 
fibrin,  a  description  of  albuminoid  material  which,  in  the  form 
in  which  it  is  found  after  death,  is  termed  syntonin,  and  is 
distinguished  from  other  varieties  of  fibrin  by  being  soluble 
in  dilute  hydrochloric  acid.     It  would  appear,  however,  that 
during  life  the  muscle-fibrin  is  in  a  fluid  condition,  and  differs 
from  the  syntonin  found  after  death ;  and  it  has,  therefore, 
been- distinguished  as  myosin.     That  a  slight  decomposition 
sets  in  soon  after  death,  seems  certain  from  the  circumstance 
that  dead  muscle  has  an  acid  reaction,  whereas  muscle  which 


56  ANIMAL    PHYSIOLOGY. 

is  still  contractile  is  neutral  or  slightly  alkaline,  except  after 
being  thrown  into  a  state  of  spasm. 

Although  muscle-fibrin  is  the  principal  solid  constituent 
of  muscle,  there  are  numerous  others  which  occur  in  small 
quantity  in  solution,  some  of  them  containing  nitrogen  and 
others  not.  The  nitrogenous  substances  referred  to  are  all 
of  them  much  simpler  in  chemical  constitution  than  the 
albuminoids,  and  the  most  important  of  them  is  called 
kreatin,  of  which  it  is  sufficient  to  note  that  it  and  its  allies 
are  of  a  composition  less  complex  than  gelatin,  and  more 
complex  than  urea.  Among  the  non-nitrogenous  substances 
found  in  muscles  may  be  mentioned  grape  sugar,  and  another 
variety  of  sugar  called  iiiosite,  also  lactic,  butyric,  and  other 
acids.  These  various  substances,  both  those  which  contain 
nitrogen  and  those  which  do  not,  are  probably  formed  by 
processes  of  decomposition  incident  to  activity  of  the  muscu- 
lar fibre,  for  their  quantity  is  greater  in  muscle  whose 
irritability  has  been  exhausted  by  electric  stimulus,  than  in 
muscle  which  has  been  at  rest.  Thus  the  hind  limbs  of  a 
frog  have  been  separated  from  the  animal,  and  one  of  them 
has  been  subjected  to  severe  electric  stimulus,  while  the  other 
has  been  left  at  rest;  and  the  muscles  of  the  stimulated 
limb  have  yielded  a  notably  larger  amount  of  substance 
soluble  in  alcohol  than  those  of  its  fellow  (Helmholtz). 

34.  During  life,  however,  muscular  action  is  sustained  by  the 
combustion  not  of  nitrogenous  material  but  of  non-nitrogen- 
ous. This  has  been  proved  by  a  variety  of  experiments,  in 
which  persons  have  been  kept  for  days  on  a  weighed  diet  of 
known  composition,  and  their  urine  and  other  excreta  have 
been  daily  analysed ;  and  it  has  been  found  that  en  days  on 
which  they  took  violent  exercise,  they  lost  no  more  nitrogen 
than  on  days  when  they  were  at  perfect  rest.  By  other  ex- 
periments it  is  known  that  the  amount  of  carbonic  acid 
given  off  by  the  lungs  is  greatly  increased  by  exercise.  A 
muscle  may,  therefore,  be  compared  with  an  engine  which 
consumes  in  its  work,  not  its  own  substance,  but  fuel.  This 
fuel  is  carbonaceous  material,  which  is  converted  with  the 
aid  of  oxygen  into  carbonic  acid  and  water;  while  it  is  in  the 
intervals  of  rest  that  the  proper  substance  of  the  muscle, 
consisting  of  albuminoid  material,  undergoes  growth  and  repair. 


MUSCLES.  57 

35.  Although  the  normal  stimulus  to  muscular  contraction 
is  derived  through  nerves,  the  muscles  may  be  excited  to 
contract  by  various  other  stimuli,  mechanical,  chemical,  and 
electrical,  and  by  heat  and  cold.  Isolated  muscular  fibres, 
both  striped  and  unstriped,  have  been  made  to  contract 
under  the  microscope.  The  irritability  of  muscular  fibre  is, 
therefore,  inherent  in  itself,  and  not  due  to  its  nervous  con- 
nections. Complete  contraction  sustained  for  a  short  time 
is  followed  by  a  condition  of  exhaustion  or  temporary  loss  of 
irritability;  but  the  duration  of  contractile  power  may  be 
greatly  increased  in  the  pathological  spasm  termed  tetanus, 
the  condition  which  constitutes  lockjaw  when  it  affects  the 
muscles  of  mastication.  However,  a  certain  slight  amount 
of  habitual  contraction  of  a  continuous  description,  distin- 
guished as  tonicity,  exists  in  a  number  of  muscles,  possibly 
in  all ;  and  we  shall  find  it  illustrated  in  the  coats  of  arteries, 
and  in  the  circular  muscles  called  sphincters,  which  keep  the 
orifices  around  which  they  are  situated  closed ;  for  example, 
the  pyloric  orifice  of  the  stomach. 

The  irritability  of  muscle  continues  for  some  time  after 
the  death  of  the  animal,  and  in  some  cold-blooded  animals 
may  continue  for  days.  The  properties  of  living  muscle  can 
therefore  be  studied  on  parts  separated  from  the  bodies  of 
animals.  A  block  of  living  muscle  placed  in  the  circuit  of  a 
galvanometer  exhibits  a  remarkable  description  of  electric 
tension  so  long  as  it  is  quiescent.  A  galvanometer  is  an 
instrument  which  indicates  the  presence  and  direction  of 
electric  currents  in  a  wire  by  means  of  the  deviations  of 
a  magnetic  needle  placed  over  an  insulated  coil.  If  the 
circuit  of  the  wire  be  completed  by  making  contact  with 
the  transversely  cut  extremities  of  the  block  of  muscle, 
or  with  points  equally  distant  from  the  centre,  no  cur- 
rent is  exhibited;  but  if  the  contact  be  made  with  any 
other  points,  the  galvanometer  indicates  the  passage  of  a 
current  through  the  wire  from  the  point  nearest  the  centre 
of  the  block  of  muscle  to  the  other;  and  this  current  is 
strongest  when  one  point  of  contact  is  at  the  centre  of  the 
block,  and  the  other  at  one  extremity.  If,  however,  the 
muscle  be  made  to  contract,  the  condition  of  electric  tension 
ceases. 


58  ANIMAL    PHYSIOLOGY. 

36.  Some  time  after  death,  at  a  period  said  to  vary  from 
ten  minutes  to  eighteen  hours,  or  longer,  a  state  of  rigidity 
of  the  muscles  sets  in,  termed  cadaveric  rigidity  or  rigor 
mortis.  It  begins  in  the  face,  and  extends  successively  to 
the  trunk  and  upper  and  lower  limbs,  and  disappears  from 
the  parts  in  the  same  order,  after  lasting  for  a  period  varying 
from  a  few  hours  to  several  days,  and  longest  in  those 
instances  in  which  it  has  set  in  latest.  It  is  longest  delayed 
and  most  marked  in  strong  subjects  who  have  been  cut  off 
in  full  vigour.  It  never  sets  in  till  after  the  disappearance 
of  irritability,  and  that  circumstance  is  sufficient  to  show 
that  it  is  not,  as  has  sometimes  been  supposed,  a  contraction 
of  the  muscles.  Rigor  mortis  is  often  so  intense  as  to  render 
it  impossible  to  alter  the  attitude  of  the  limbs  without  tear- 
ing the  muscles  or  damaging  the  bones;  but  it  never  alters 
the  position  in  which  the  body  is  lying;  for  example,  it  does 
not  raise  the  jaw  when  it  has  dropped  in  death;  and  in  this 
it  differs  obviously  from  muscular  contraction.  The  use  of 
muscles  is  to  produce  movement  by  their  contraction;  and 
when  two  opposing  groups  are  both  made  spasmodically  rigid, 
as  may  happen  in  tetanus,  the  stronger  overcomes  the  other. 
But  so  far  from  this  being  the  case  in  rigor  mortis,  it  is  known 
to  every  undertaker  that  a  body  stiffens  in  whatever  position 
it  is  placed  in.  No  doubt  some  apparently  well  authenticated 
stories  are  on  record  of  movements  of  the  limbs  taking  place 
in  persons  who  had  died  of  yellow  fever;  but,  however  difficult 
such  cases  may  be  to  account  for,  the  very  circumstance  that 
the  movements  were  neither  spasms  nor  mere  twitchings,  but 
of  a  combined  description,  shows  that  they  were  not  a  variety 
of  rigor  mortis,  and  did  not  originate  in  the  muscular  texture, 
but  in  the  nerves;  and  the  explanation  of  them  must  be 
sought  in  some  irritation  of  the  central  nervous  system, 
probably  by  a  product  of  decomposition,  before  irritability 
had  ceased  in  the  muscles.  It  appears,  then,  that  rigor  mortis 
produces  no  change  in  the  length  of  the  muscles ;  and  there 
seems  good  reason  to  accept  the  hypothesis  that  it  is  a 
phenomenon  due  to  chemical  change  which  coagulates  the 
myosin;  but  it  must  be  admitted  that  we  are  not  yet  pro- 
perly acquainted  with  the  chemical  distinctions  of  muscle 
prior  to,  during,  and  subsequent  to  rigor  mortis. 


MUSCLES. 


59 


37.  Unstriped  muscle  is,  for  the  most  part,  found  diffused 
among  the  other  tissues  of  organs,  into  the  formation  of  which 
it  enters,  or  lying  in  strata.  Striped  muscle,  for  the  most 
part,  is  gathered  into  definitely  limited  organs  called  muscles, 
which  have  a  distinct  origin  and  insertion,  and  usually  a 
certain  amount  of  tendon  in  their  construction.  The  extent 
of  action  of  a  muscle  depends  on  the  length  of  its  fibres, 
while  its  strength  of  action  is  in  proportion  to  the  number  of 
them.  When  therefore  a  muscle  consists 
of  few  fibres  running  lengthwise  from  origin 
to  insertion,  it  is  useful  rather  for  the  extent 
of  the  movement  which  it  serves,  than  for 
the  force  which  it  gives  to  it.  But  when 
great  resisting  power  is  required  in  a  muscle, 
its  tendons  extend  through  its  substance,  and 
its  muscular  fibres  are  short  and  numerous, 
passing  obliquely  from  one  prolongation  of 
tendon  to  another:  thus  in  the  soleus,  one 
of  the  muscles  of  the  calf  already  referred 
to  (p.  51),  the  fibres  are  arranged  in  four 
oblique  sets,  and  are  none  of  them  more  than 
about  an  inch  in  length;  so  that  the  muscle 
is  incapable  of  approximating  its  attach- 
ments more  than  an  inch,  but  can  exert  in 
its  limited  range  an  enormous  force,  and  is 
thus  well  adapted  to  sustain  the  weight  of 
the  body  in  standing. 


Fig.  32.  —  Deep 
surface  of  so- 
leus muscle. 


CHAPTER  V. 

FREE  SURFACES,  EPITHELIUM,  SECRETION,  AND 
INTEGUMENT. 

38.  THE  free  surfaces  to  be  noted  in  the  study  of  the  body 
are  of  three  descriptions  : — 

First,  the  external  surface  of  the  body  is  covered  with 
integument. 

Secondly,  the  surfaces  of  hollows  and  passages  communi- 
cating freely  with  the  outside,  such  as  the  alimentary  canal 
and  the  ducts  of  glands,  are  lined  with  mucous  membrane, 
so  named  from  the  mucus  with  which  it  is  lubricated,  thrown 
out  from  its  surface,  or  supplied  by  glands. 

Thirdly,  the  surfaces  of  cavities  and  canals  which  have 
little  or  no  communication  with  the  outside,  have  a  smoothly 
polished  appearance,  and  from  them  transudes,  in  the  case  of 
empty  cavities,  a  sufficient  amount  of  fluid  to  moisten  them. 
This  group  of  surfaces  includes  serous  membranes,  which  are 
shut  sacs  of  delicate  membrane,  extending  over  the  surfaces 
of  viscera,  and  lining  the  opposing  walls  of  the  cavities 
which  contain  them,  so  as  to  allow  free  gliding  movement; 
thus  the  abdominal  viscera  and  the  opposed  walls  of  the 
abdomen  are  invested  with  the  peritoneum,  the  lungs  and 
ribs  with  the  right  and  left  pleura,  and  the  heart  with  the 
pericardium.  The  synovial  membranes  lining  the  cavities 
of  joints  are  similar  to  serous  membranes,  but  are  not  con- 
tinued over  the  faces  of  the  articular  cartilages.  Synovial 
bursce,  provided  for  the  free  gliding  of  muscles  or  integument 
over  bone,  such  as  the  bursa  on  the  front  of  the  knee,  the  dis- 
tention  of  which  with  fluid  constitutes  the  common  affection 
called  housemaid's  knee,  are  similar  sacs  with  more  slender 
walls;  as  also  are  the  sheaths,  called  thecm,  in  which  a 
number  of  tendons  glide.  So  also  the  interior  of  blood- 


EPITHELIUM. 


Gl 


vessels  and  lymphatics  are  polished  and  smooth,  and  tho 
interior  of  the  heart  is  lined  with  membrane  very  similar  to 
the  pericardium  round  it- 

39,  Epithelium. — All  the  free  surfaces  now  alluded  to 
agree  in  one  point,  namely,  that  they  are  clothed  with  one 
or  more  strata  of  nucleated  corpuscles.  Such  investments 
are  termed  epithelium,  and  the  corpuscles  which  compose 
them  epithelial  cells. 

Epithelia,  in  which  the  cells  are  arranged  in  a  single  layer, 
are  termed  simple,  while  those  in  which  there  are  several 
layers,  are  distinguished  as  stratified;  and  stratified  epithelia 
are  named  according  to  the  character  of  the  superficial  layer, 
without  regard  to  the  numerous  shapes  of  corpuscles  in  the 
deep  layers. 

In  squamous  or  pavement  epithelium  the  cells  are  flattened 
like  scales.  The  surface  belonging  to  the  third  group  above 
described,  including  serous  and  sy  no  vial  membranes,  and  the 
interiors  of  vessels,  are  all  clothed  with  simple  squamous 
epithelium  of  delicate  microscopic  character.  The  cuticle, 
which  will  be  more  particularly  described  anon,  and  the 
mucous  membranes  of  the  mouth,  eyelids,  and  urinary 


Fig.  S3.— VARIETIES  OF  EPITHELIUM,  a,  separate  squamous  cells 
from  mouth ;  6,  from  serous  membrane ;  c,  fluted  cell  from  deep 
part  of  epidermis ;  d,  columnar  cells  of  intestine ;  e,  cubical  cells 
of  kidney;  /,  ciliated  columnar  stratified  epithelium  of  wind- 
pipe ;  g,  ciliated  spherical  cells  from  choroid  plexus  of  brain. 


62  ANIMAL   PHYSIOLOGY. 

bladder,  afford  instances  of  stratified  squamous  epithelium. 
The  microscopic  examination  of  the  more  delicate  simple 
squamous  epithelia,  is  facilitated  by  treating  the  surface  to 
be  examined  with  a  solution  consisting  of  one  grain  of  nitrate 
of  silver  in  an  ounce  of  distilled  water,  and,  after  a  few 
minutes,  washing  it  and  exposing  to  the  light.  Oxide  of 
silver  is  deposited,  first  in  the  lines  of  contact  of  the  edges 
of  the  cells,  and,  on  a  little  further  exposure,  also  in  their 
substance,  particularly  in  the  nuclei. 

Columnar  epithelium  has  the  cells  elongated  in  a  direction 
vertical  to  the  surface,  and  lying  together  like  rods  or  prisms. 
It  is  found  in  the  whole  length  of  the  alimentary  canal,  from 
the  entrance  into  the  stomach  and  onwards,  and  in  the 
majority  of  ducts  of  glands.  In  glands  and  their  ducts, 
there  are  also  various  transitional  forms  between  columnar 
and  squamous,  as,  for  example,  the  cubical ;  and  the  terms 
spheroidal  epithelium  and  glandular  are  used  for  the  various 
irregular  polygonal  forms  of  corpuscle  engaged  in  secretion 
in  the  salivary  and  other  glands. 

Ciliated  epithelium  is  neither  the  mere  mechanical  protec- 
tion which  squamous  epithelium  is,  nor  the  secreting  struc- 
ture which  columnar  and  others  often  are,  but  has  the 
property  of  keeping  the  moisture  on  its  surface  in  a  perpetual 
current.  This  it  does  by  means  of  minute  hair-like  processes, 
termed  cilia,  projecting  from  the  free  aspect  of  its  cells,  and 
perpetually  in  motion.  In  a  suitable  fluid  these  cilia  will 
continue  to  move  for  hours  under  the  microscope,  after  the 
cells  to  which  -they  belong  have  been  detached  from  all  other 
texture  ;  and  therefore  both  the  power  of  movement  or  con- 
tractility, and  the  stimulus  thereto,  must  be  inherent  in 
themselves.  The  character  of  the  movement  is  always  the 
same ;  it  is  so  rapid  that  it  cannot  be  observed  in  detail  till 
it  begins  to  get  slower,  but  the  general  effect  is  something 
like  the  undulation  of  a  field  of  corn  swept  by  the  wind,  or 
still  liker  the  vibration  of  hot  air  over  a  furnace  seen  against 
the  light.  When  the  rapidity  abates,  each  cilium  is  seen  to 
be  slightly  flattened,  and  to  bend  over  to  one  side,  and 
recover  with  a  feathering  curve.  The  movement  is  at  all 
times  in  one  direction,  and  that  direction  is  in  every  case 
toward  the  orifice  of  the  passage,  when  there  is  one.  Ciliated 


SECRETION1.  63 

epithelium,  with  tlie  cells  of  a  columnar  form,  is  found  cloth- 
ing the  respiratory  passages,  including  the  lower  part  of 
the  nasal  cavity,  the  larynx,  trachea,  and  bronchial  tubes ; 
it  also  lines  the  Eustachian  tubes,  and  parts  of  the  repro- 
ductive passages,  both  male  and  female.  Ciliated  epithelium 
of  spheroidal  character  is  common  in  the  invertebrate  and 
lower  vertebrate  animals,  and  is  found  in  the  ventricles  of 
the  human  brain. 

40.  Secretion. — It  has  been  pointed  out  that  epithelial 
cells  are  in  some  instances  devoted  to  secretion,  and  it  will 
be  convenient  in  this  place  to  explain  the  nature  of  that 
function.  Secretion  is  the  preparation  and  separation  of  any 
substance  from,  the  economy;  and  the  substance  may  either 
pre-exist  in  the  blood  and  be  merely  extracted  therefrom,  or 
it  may  be  a  new  substance  elaborated  by  the  secreting  organ. 
Thus,  in  the  perspiration  there  is  little  which  does  not  pre- 
exist in  the  blood,  while  in  the  bile  the  most  important  in- 
gredients not  only  are  not  constituents  of  the  blood,  but  are 
poisonous  when  absorbed  into  it.  In  probably  all  instances, 
nucleated  corpuscles  of  the  description  of  epithelial  cells  are 
agents  in  secretion.  The  water  contained  in  some  secretions 
may  escape  from  the  blood  in  part  by  mere  percolation,  but 
the  solid  constituents  are  selected  or  manufactured  by  the 
secreting  corpuscles  or  cells. 

A  certain  amount  of  serous  or  mucous  secretion  may 
escape  from,  the  general  surface  of  serous  or  mucous  cavities  j 
but  for  the  production  of  special  secretions  there  are  glands 
provided,  which  are  organs  composed  essentially  of  recesses 
opening  off  from  epithelial  surfaces  and  lined  with  secreting 
cells,  which  obtain  the  material  on  which  they  act  from  a 
copious  supply  of  blood-vessels  round  about.  The  recesses 
of  which  glands  consist  may  be  either  tubules  or  saccules, 
which  may  open  singly  on  a  free  surface,  constituting  simple 
glands,  or  may  be  gathered  into  groups,  forming  compound 
glands,  which,  whether  tubular  or  saccular  in  the  secreting 
part,  pour  their  secretion  into  tubules  converging  to  a 
common  duct.  Compound  sacculated  glands  have  the 
primitive  saccules  arranged  in  lobules,  and  these  in  larger 
lobules ;  and  such  glands  are  therefore  termed  lobulated. 

With  respect  to  the  mode  of  action  of  the  secreting  cells 


ANIMAL   PHYSIOLOGY. 


or  nucleated  corpuscles,  it  may  be  remarked  that,  in  the  in- 
stance of  one  of  the  salivary  glands  (the  sub-maxillary)  in 


Fig.  34. — LOBULE  OF  LIVER  OF      Fig.    35. — LOBULE    OF   PAROTID 
OYSTER.  GLAND  OF  EMBRYO  LAMB  five 

-inches  long. 

the  rabbit  and  the  ox,  branches  of  nerve  have  been  traced 
under  the  microscope  to  the  individual  corpuscles,  and  it  has 
been  found  that  stimulation  of  the  chorda  tympani  nerve 
supplying  the  gland  excites  secretion,  and  that  the  corpuscles 
of  a  gland  so  excited  have  a  different  appearance  from  those 
of  a  gland  which  has  been  at  rest,  losing  the  sharply  denned 
transparent  and  slightly  striated  appearance  which  they 
have  when  at  rest,  the  contour  lines  becoming  indistinct, 

and  the  substance  altered,  so  as 
to  be  capable  of  being  stained 
more  uniformly  with  carmine 
(Heidenhain  and  PMger).  So 
also  the  secreting  cells  of  the 
glands  of  the  stomach  have 
been  found  to  have  a  different 
appearance  during  digestion  from 
what  they  have  in  an  unfed 
animal,  but  no  appearance  of 
multiplication  of  the  cells  at 
these  times  can  be  detected.  It 
therefore  appears  that  the  se- 


Fig.  36.  —  SECRETING  COR- 
PUSCLES of  sub -maxillary 
gland  of  the  ox  with  nerve- 
fibres  ending  in  them.  After 
Pfiiiger. 


creting  cells  act  by  undergoing,  under  nervous  stimulation,  a 


THE    INTEGUMENT. 


65 


change  of  condition,  during  which  they  have  powers  of  attrac- 
tion, elaboration,  and  transmission  of  substances.  The  active 
condition  of  the  cells  is  not,  however,  proved  to  be  in  all 
instances  occasional,  and  excited  only  by  nervous  stimulus. 
While  saliva  and  gastric  juice  are  poured  out  in  response  to 
occasional  nervous  irritations,  the  secretions  of  the  kidneys 
and  skin  never  wholly  cease  in  health;  and  it  is  possible 
that  in  these  organs  the  corpuscles  have  a  certain  amount  of 
continuous  activity,  comparable  with  tonicity  in  muscles. 

41.  The  Integument. — The  integument  combines  the 
functions  of  protection,  sensation,  and  secretion,  and  consists 
of  two  parts,  the  epidermis  and  the  cutis  vera. 


Fig.  37. — INTEGUMENT  OF  HAND,  vertical  section  magnified,  a  c, 
epidermis;  a  6,  horny  layer;  b  c,  rete  mucosum;  c,  elongated 
corpuscles  of  deepest  layer  ;  d  d,  capillary  blood-vessels  in  two 
papillae ;  e,  nerve  fibre,  ending  in  a  touch  corpuscle ;  /,  duct  of 
sweat  gland,  spiral  in  the  horny  layer ;  g,  the  same,  beneath  the 
integument. 

The  epidermis }  cuticle,  or  scarf-skin,  is  a  stratified  squamous 
14  E 


66  ANIMAL   PHYSIOLOGY. 

epithelium.  When  very  thin  sections  vertical  to  the  surface 
are  made  through  it,  and  examined  under  the  microscope, 
it  is  seen  to  consist  of  two  parts,  which  are  very  different  in 
appearance :  a  deep  part  consisting  of  delicate  texture,  and  a 
superficial  which  is  horny.  The  difference  may  be  made 
very  striking  by  acting  on  the  specimen  with  a  drop  of  an 
ammonia  solution  of  carmine,  which  takes  110  effect  on  the 
horny  part,  but  stains  the  deep  part,  and  particularly  the 
nuclei,  it  being  the  property  of  that  solution  so  to  act  on 
all  growing  nucleated  corpuscles. 

The  deep  part  of  the  cuticle  is  likewise  called  the 
mucous  layer  (rete  mucosum,  or  Malpighian  layer),  and 
it  presents  several  strata  of  cells.  Those  which  lie 
deepest,  resting  on  the  cutis  vera,  are  always  somewhat 
elongated  vertically,  while  those  immediately  superficial 
to  these  are  small,  and  the  remaining  strata  exhibit 
cells  larger  and  more  flattened  the  nearer  they  are  to 
the  surface.  It  appears,  therefore,  that  whatever  may  be 
the  mode  of  origin  of  the  deepest  or  elongated  cells,  the 
other  layers  consist  of  elements,  the  history  of  each  of  which 
is,  that  it  has  originated  as  one  of  the  minutest  cells,  and 
passes  gradually  to  the  surface  as  it  enlarges,  undergoing 
both  change  of  shape  and  chemical  composition,  until  it 
becomes  incorporated  with  the  horny  part  of  the  cuticle,  and 
is  ultimately  shed  from  the  surface  in  the  shape  of  a  small 
scale. 

The  superficial  or  horny  part  of  the  cuticle  consists  of 
flattened  cells  closely  adherent  one  to  another,  receiving 
additions  from  the  mucous  layer  on  the  deep  side,  and  cast- 
ing off  its  oldest  cells  from  the  suiface  in  a  perpetual  in- 
sensible desquamation.  Its  cells  may  be  separated  and  their 
nuclei  displayed  by  the  action  of  a  solution  of  caustic  potash. 
The  explanation  of  blistering  is,  that  the  mucous  layer,  acted 
on  by  some  unwonted  irritation,  pours  out  a  serous  discharge, 
which,  being  pent  up  by  the  impermeable  horny  part  of  the 
cuticle,  separates  it  from  its  connections. 

The  cuticle,  besides  the  mechanical  protection  which  it 
gives  the  body,  furnishes  likewise,  by  the  impermeability  of 
its  horny  layer,  and  the  intervention  of  living  parts  between 
t^p  surface  ajad  the  vascular  channels  within,  a  protection 


THE   INTEGUMENT.  67 

against  tlie  absorption  of  poisons,  so  that  these,  as  is  familiar 
to  every  one,  may  be  handled  with  impunity  when  the  cuticle 
is  perfect,  and  yet  may  be  introduced  into  the  system  by  a 
very  small  wound.  The  surface  of  the  body  is  not,  however, 
to  be  supposed  incapable  altogether  of  absorbing  substances 
from  without.  Fluids  left  in  contact  with  the  cuticle,  and 
even  solid  substances  rubbed  into  it,  are  gradually  absorbed. 
Probably  such  absorption  takes  place  principally  in  the  orifices 
of  the  sweat  glands,  but  even  these  are  lined  with  thin  pro- 
longations of  the  horny  cuticle,  which  therefore  cannot  be 
regarded  as  totally  impermeable. 

A  variable  amount  of  pigment  exists  in  the  deep  cells  of 
the  cuticle,  and  it  is  on  this  that  all  duskiness  of  skin 
depends,  the  greatest  amount  of  pigment  being  in  the  cuticle 
of  the  negro.  Among  the  chemical  changes,  however,  which 
take  place  in  the  epidermal  cells  as  they  approach  the  sur- 
face, is  the  disappearance  of  the  pigment;  and  the  horny 
layer  is  pale  which  is  raised  by  a  blister  from  the  negro's 
skin.  The  red  tints  of  the  skin  are  not  dependent  on  pig- 
ment, but  011  the  blood  shining  through  from  the  blood- 
vessels in  the  cutis  vera. 

42.  The  cutis  vera,  derma,  corium,  or  true  skin,  has  a  white 
fibrous  basis.  Its  surface  is  thrown  into  papillae  or  finger- like 
prominences,  the  largest  of  which  are  about  j^-  of  an  inch 
in  length.  In  the  papillary  part,  it  is  impossible  to  detect 
separate  fibres ;  while  in  the  deeper  part,  the  white  fibrous 
substance  is  arranged  in  interlacing  bands,  the  spaces  between 
which  get  larger  as  the  subcutaneous  tissue  is  approached, 
into  which  the  skin  gradually  blends.  In  the  deep  part  also, 
elastic  fibres  curl  and  twine  in  all  directions,  and  there  is  a 
copious  network  of  connective-tissue-corpuscles  with  long 
processes.  The  superficial  part  is  much  more  vascular  than 
the  deep ;  for  close  to  the  surface  of  the  cutis  is  spread  one 
of  the  richest  networks  of  capillary  blood-vessels  in  the  body, 
with  a  loop  of  blood-vessel  in  every  papilla.  It  is  from  these 
blood-vessels  that  the  epidermis  receives  its  nourishment. 
The  epidermis  is  moulded  to  the  papillary  surface  of  the 
cutis ;  and  in  the  hands  and  soles  of  the  feet,  which  have  the 
papillae  disposed  in  thickest  rows,  the  trace  of  this  arrange- 
ment is  left  oil  the  surface  of  the  cuticle,  in  the  form  of  the 


68  ANIMAI   PHYSIOLOGY 

ridges  with  furrows  between  them  which  characterise  those 
parts. 

43.  The  sensibility  of  the  skin  is  due  to  the  presence  of 
nerve  terminations,  which  are  of  different  descriptions  and 
at  different  depths.    The  largest  of  these  are  termed  Pacinian 
bodies,  and  are  especially  found  in  the  subcutaneous  adipose 
tissue  of  the  fingers  and  toes  (fig.   38).      They  are  grape- 
shaped  structures  of  such  size  that  they  can  be  recognised 
by  the  practised  dissector  with  the  naked  eye  as  minute 
grains,  being  upwards  of  —^  of  an  inch  in  length  ;  and  they 
consist  each  of  a  dilated  end  of  a  nerve  fibre,  with  layers 
of   tough  nucleated    tissue   round    about.      They   are   not 
peculiarly  integumentary  structures,  for  the  site  in  which 
above  all  others  they  are  found  easily  and  abundantly,  is 
the  mesentery  of  the  lower  bowel  of  the  cat.     Within  a 
number  of  the  papillae  of  the  cutis  vera  smaller  bodies  are 
found,  termed  touch-corpuscles  of  Wagner  (fig.  37).     These 
are   of  such  size,  that   each   one  fills  the   greater  part  of 
the  papilla  in  which  it  is  contained  :  the  structure  consists 
of  a  firm  nucleated  core,  round  which  the  nerve  is  coiled. 
Still  smaller  end-bulbs  (of  Krause)  are  found  in  or  beneath 
the  papillae  in  places  where  the  skin  is  delicate,  as  on  the 
lips  and  over  the  white  of  the  eye,  and  appear  to  resemble 
the  Pacinian  bodies  in  having  the  nerve  ending  in  the  in- 
terior.    Lastly,  it  is  to  be  noticed  that,  independently  of  all 
these  modes  of  nerve  termination,  nervous  filaments  have 
been  found  ramifying  between  the  cells  of  the  epidermis,  and 
possibly  terminate  in  individual  cells ;  and,  although  this  is 
the  most  difficult  method  of  nerve-termination  to  trace,  there 
can  be  little  doubt  that  it  is  the  most  important. 

44,  The  glands  of  the  skin  are  of  two  descriptions,  the 
sudoriparous  and  the  sebaceous. 

The  sudoriparous,  or  sweat  glands,  are  in  great  numbers 
all  over  the  body.  In  the  palm  of  the  hand  there  are  as 
many  as  2500  in  every  square  inch  of  skin,  but  in  the  lower 
limbs  and  back  there  have  been  estimated  to  be  not  more 
than  600  in  the  square  inch.  On  the  palm,  particularly 
when  it  is  warm  and  slightly  moist,  the  orifices  of  these 
glands  may  be  easily  seen  with  a  simple  pocket  lens  arranged 
in  a  row  on  every  ridge.  Each  gland  consists  of  a  tubule, 


(THE  GLANDS.  69 

the  secreting  part  of  which  is  coiled  up  into  a  ball  about  -£$ 
or  T^j-  inch  in  diameter,  imbedded  in  the  subcutaneous 
tissue,  and  supplied  freely  with  blood-vessels,  while  the  duct 
passes  vertically  up  through  the  corium.  Sometimes  there 
are  two  tubes  in  one  coil;  uniting  to  form  one  duct.  In  the 


Fig.  38.— SWEAT  GLANDS  AND  PACTNIAN  CORPUSCLES,  a,  Papillary 
surface  of  corium ;  b,  secreting  portions  of  the  sweat  glands ;  c, 
corpuscles  of  Pacini.  _w  „ 

palm  of  the  hand  and  the  sole  of  the  foot,  where  the  epider- 
mis is  thick,  the  sudoriparous  ducts,  in  passing  through  the 
horny  layer,  are  thrown  into  a  fine  spiral  like  a  cork-screw, 
which  may  be  accounted  for  thus:  the  ducts  in  the  deep 
part  of  the  cuticle  are  lined  with  horny  epithelium,  which  is 
incapable  of  contraction,  while  round  about  them  are  grow- 
ing epithelial  cells  of  the  rete  mucosum,  which,  as  they 


70  ANIMAL   PHYSIOLOGY. 

approach  the  surface,  in  process  of  conversion  into  the  horny- 
layer,  become  flattened ;  the  horny  wall  of  the  duct,  as  it  is 
carried  to  the  surface  with  the  structures  in  which  it  is  im- 
bedded, having  therefore  to  be  accommodated  in  a  shorter 
vertical  depth  than  it  occupied  at  first,  is  pressed  by  the 
shrinking  structures  round  it  into  an  oblique  position,  and 
the  regular  continuance  of  this  process  gives  rise  to  a  spiral. 

The  sebaceous  glands  (fig,  41)  are  for  the  most  part  con- 
nected with  hairs ;  and  large  hairs  have  usually  several  of 
them  opening  into  the  necks  of  their  follicles,  but  fre- 
quently it  happens,  particularly  in  the  face,  that  a  very 
large  sebaceous  gland  opens  into  the  neck  of  the  follicle  of 
a  very  small  hair,  showing  that  these  glands  not  only  serve 
to  lubricate  the  hair,  but  the  integument  as  well.  The 
sebaceous  glands  consist  of  one  or  more  small  groups  of 
saccules  opening  into  a  common  duct,  lined  with  epithelium, 
and  filled  with  oily  matter, 

45.  The  perspiration  is  the  combined  product  of  both  sets 
of  glands,  but  principally  is  derived  from  the  sudoriparous. 
The  sebaceous  glands  secrete  nothing  but  oil,  and  they  are 
not  the  exclusive  source  of  the  oil  of  the  skin ;  for  not  only 
may  particles  of  oil  be  detected  in  the  interior  of  large  sweat 
glands,  but  oil  is  secreted  by  the  palms  of  the  hands,  which 
have  no  sebaceous  glands.  In  connection  with  this  it  may 
be  mentioned  that  the  ceruminous  glands,  which  secrete  the 
wax  of  the  ears,  are  simply  large  sweat  glands.  The  most 
abundant  solid  constituent  of  the  perspiration  is  common, 
salt,  chloride  of  sodium;  and  besides  other  salts,  there  is 
always  a  small  amount  of  urea  in  it.  Carbonic  acid  is 
likewise  given  off  by  the  skin,  although  the  amount  of  it 
is  insignificant  compared  with  what  escapes  by  the  lungs. 
The  total  amount  of  perspiration  is  obviously  exceedingly 
variable;  but  in  experiments  made  by  enveloping  the  body 
in  a  water-tight  bag  with  apertures  for  breathing, -it  has  been 
found  to  be  little  short  of  two  pounds  per  diem.  One  obvious 
use  of  the  perspiration,  and  probably  the  principal  purpose 
which  it  serves,  is  the  protection  of  the  body  from  too  great 
heat,  whether  of  external  or  internal  origin,  by  the  cooling 
effect  of  its  evaporation  from  the  surface,  as  will  be  further 
referred  to  (p.  149).  Its  flow,  like  that  of  saliva,  is  probably 


EPIDERMAL  APPENDAGES.  71 

under  the  control  of  the  nervous  system  ;  certainly  it  is  no 
mere  filtration  dependent  only  on  the  amount  of  blood  sent 
to  the  surface,  for  the  skin  may  be  hot  and  dry,  particularly 
in  fevers,  and  a  cold  sweat  may  burst  out  when  the  surface 
is  pale  from  deficient  flow  of  blood  to  the  surface,  as  in  the 
recovery  from  fainting. 

Considering  the  function  of  the  perspiration  as  a  moderator 
of  the  temperature  of  the  body,  the  results  which  are  obtained 
by  varnishing  the  bodies  of  animals  with  an  impermeable 
coating  are  not  only  interesting,  but  exceedingly  difficult  to 
explain.  In  such  experiments,  when  the  varnishing  is  com- 
plete, the  temperature  rapidly  falls,  and  the  animals  die  after 
periods  varying  from  a  few  hours  to  days;  the  smaller 
animals,  those  in  which  the  total  surface  bears  the  largest 
proportion  to  the  body,  being  thosa  which  succumb  soonest. 
Why  the  temperature  falls  is  not  understood ;  but  whatever 
the  cause  of  death,  such  experiments  show  the  importance, 
in  a  sanitary  point  of  view,  of  removing  accidental  accumu- 
lations of  all  kinds  from  the  surface  of  the  body. 

46.  Epidermal  Appendages. — For  purposes  of  protection 
there  occur  in  different  animals  a  variety  of  special  growths 
from  the  cuticle ;  and  those  which  occur  in  the  human  sub- 
ject are  nails  and  hairs. 


Fig.  39, — NAIL  AND  ITS  MATRIX,  longitudinal  section,      a,  Horny 
layer  of  epidermis  ;  &,  mucous  layer  ;  c,  corium.        ...„  ... 

A  Nail  is  simply  a  thickening  of  the  outer  layer  of  the 
cuticle  growing  from  a  bed  or  matrix,  which  is  in  the  form 
of  a  fold  at  the  back  part.  In  this  fold  there  are  two  sur- 
faces of  skin  looking  one  toward  the  other,  and  thus  the  root 
of  the  nail  receives  additions  from  above  and  below,  as  well 
as  behind.  It  is  pushed  continually  forwards  by  new  growth 
at  the  bottom  of  the  fold,  and  continues  to  receive  additions 
to  its  thickness  from  the  flat  part  of  the  matrix  as  long  as  it 


?3  ANIMAL   PHYSIOLOGY. 

is  adherent  thereto.  Hence  it  happens  that  the  nail  is 
stronger  the  farther  from  the  root,  and  that  its  outer  surface 
is  harder  than  the  deep  part  where  the  recent  and  soft  addi- 
tions to  the  thickness  are  placed. 

The  substance  of  a  nail  is  an  instance  of  the  texture  called 
horn.  But  it  is  always  to  be  kept  in  mind  that  the  word 
horn  has  a  double  meaning,  which  has  descended  to  it  from 
the  Latin.  This  has  obviously  arisen  from  the  structure 
of  the  horns  of  the  sheep  and  cxen,  which  consist  of  an  outer 
coating  of  the  texture  called  horn  investing  a  core  of  bone. 
But  on  the  horn  of  the  stag  there  is  no  horny  covering,  the 
structure  being  a  growth  of  bone,  which,  when  young,  is 
covered  with  integument,  but  afterwards  becomes  denuded. 
Instances  of  morbid  growth  of  solid  horny  texture  occur 
occasionally  in  man,  but  rarely  grow  to  any  considerable  size. 
In  bedridden  persons,  whose  nails  have  been  neglected,  it 
sometimes  happens  that  some  of  them  project  like  claws, 
curving  over  beyond  the  digit,  and  become  nearly  as  thick  as 
they  are  broad. 

47.  A  Hair  consists  of  a  bulb  or  root  imbedded  in  a  follicle, 
and  a  shaft  or  stem  ascending  therefrom.  The  follicle  is  an 
invagination  of  the  integument  lined  with  a  thin  prolongation 
of  the  cuticle,  divisible  into  two  layers,  sometimes  called  the 
inner  and  outer  root-sheath,  arid  so  adherent  to  the  root  of 
the  hair  that  it  is  liable  to  be  removed  with  it  when  a  hair  is 
pulled  out.  At  the  base  of  the  follicle  is  an  enlarged 
papilla ;  and  the  hair  itself  may  be  considered  as  the  ex- 
aggerated cuticular  investment  of  this  papilla.  The  root 
of  the  hair  forms  a  bulbous  enlargement  round  the  papilla, 
and  consists  in  greater  part  of  polygonal  cells,  which  in 
dark  hairs  are  loaded  with  pigment ;  but  towards  the  upper 
part,  round  about,  the  cellular  substance  is  changed  to 
iibrous,  and  on  the  surface  there  is  an  imbricated  epithelium 
continuous,  below,  with  the  innermost  layer  of  'the  cuticular 
lining  of  the  follicle.  All  these  three  elements  may  be  repre- 
sented in  the  stem.  The  epithelium  on  the  surface  of  the 
root,  traced  upwards,  is  seen  to  form  on  the  stem  an  ex- 
tremely thin  coat,  of  which  the  most  easily  discernible  part 
is  a  network  formed  by  the  thickened  edges  of  the  scales.  In 
many  of  the  larger  hairs,  the  cells  which  constitute  the  main 


EPIDERMAL   APPENDAGES. 


73 


bulk  of  tlie  root  are  con- 
tinued in  a  column  up  the 
centre  of  the  stem,  and  are 
termed  the  medulla.  But  in 
the  smaller  hairs  of  the  body, 
and  even  often  in  the  hairs 
of  the  head,  the  medulla  is 
absent,  or  only  present  in 
small  patches ;  and  in  all 
instances  the  bulk  of  the 
stem  is  of  fibrous  substance, 
which,  in  contradistinction  to 
the  medulla,  is  termed  the 
cortical  part.  This  cortical 
substance  appears  in  the 
natural  state  nearly  homo- 
geneous; but  when  boiled 
with  potash  it  is  seen  to 
consist  of  flat  fibres,  which 
are  derived  from  the  cells 
of  the  root  by  elongation 
and  alteration  of  consistence. 
The  three  elements  of  the 
stem  of  the  hair  are  very  dif- 
ferently represented  in  differ- 
ent animals.  In  wool  the 
edges  of  the  epithelial  cells  are 
prominent,  and  by  their  ten- 
dency to  catch  cause  the  hairs 
to  be  easily  felted.  In  some 
small  animals  the  epithelium 
is  like  a  series  of  hcllow  cones 
embracing  the  hairs.  In  some, 
as  in  the  mouse,  the  medulla 
is  thrown  into  a  series  of  air 

Fig.  40. — HAIR,  a,  Papilla  in  the  centre  of  the  bulb  ;  5,  dermic 
coat  of  the  hair-follicle ;  c,  d,  outer  and  inner  cuticular  lining  ; 
e,  cortex  of  the  shaft ;  /,  medulla  ;  g,  epithelium ;  g',  the  same 
seen  in  profile,  imbricated  on  the  root ;  h,  portion  of  shaft  be- 
come white,  showing  the  medulla  enlarged  by  presence  of  minute 
air-bells,  reflecting  the  light,  and  making  the  hair  thicker. 


74  ANIMAL   PHYSIOLOGY. 

cavities,  and  in   others,  sncli  as   the   pig,  the  medulla   is 
entirely  absent. 

48.  The  shaft  of  the  hair  is  in  some  instances  cylindrical,  and 
in  others  flattened,  and  the  tendency  to  curling  is  connected 
with  the  form.  Thus  the  straight  hair  of  the  North  American 
Indian  is  cylindrical,  and  the  negro's  woolly  hair  is  quite  flat. 
The  pigment,  on  which  the  colour  depends,  is  diffused  in 
variable  degree  through  cortex  and  medulla ;  but  the  most 
curious  point  with  regard  to  the  medulla  is  its  connection 
with  the  turning  of  the  hair  white.  In  that  change  there  is 
a  disappearance  of  pigment;  but  there  is  likewise  the  develop- 
ment in  the  medulla  of  numbers  of  closely  set  minute  globules 
of  air.  Such  air  globules  are  also  constantly  present  in  the 
white  hairs  of  other  animals,  and  reflect  the  light  from  their 
surfaces.  A  hair  may  begin  to  turn  white  in  any  part  of  its 
course,  or  in  patches  at  different  points ;  and  sometimes  one 
may  see  in  one  hair  some  parts  unchanged,  some  with 
the  pigment  gone,  but  without  air  globules,  others  with 
air  globules,  but  with  the  pigment  remaining,  and  parts  with 
both  changes  complete.  It  will  be  observed,  therefore,  that 
the  hair  undergoes  changes  of  nutrition  in  its  whole  length. 


.  41. — SECTION  or  SCALP,  showing  two  roots  of  hairs,  a,  a, 
Blood-vessels;  &,&,  erector  muscles  of  the  hair;  c,c,  sebaceous 
glands. 


EPIDERMAL   APPENDAGES. 


These  may  take  place  with  great  rapidity ;  for  at  least  one 
case  has  occurred  in  hospital,  under  medical  supervision,  in 


in  a  single 


night  from 


which  the  hair  has  grown  white 
anxiety,  and  there  is,  therefore,  no 
reason  to  doubt  the  historical  traditions 
of  similar  occurrences.  The  appearance 
in  one  night  also  of  single  white  hairs 
without  any  special  disturbance  of  the 
system,  has  been  noted  by  competent 
observers,  and  is  probably  a  very  com- 
mon occurrence. 

The  sensation  of  the  hair  standing 
erect  from  emotional  or  other  causes  is 
accounted  for  by  the  fact  that  a  band 
of  unstriped  muscle  descends  from  tho 
corium,  and  is  attached  to  the  lower 
part  of  the  hair  follicle  on  the  side 
towards  which  the  hair  is  sloped,  so 
that  by  its  contraction  it  pulls  the  root 
of  the  hair  into  the  vertical  position. 

The  first  commencement  of  a  hair  in 
the  embryo  consists  of  a  thickening  of 
the  cuticle  by  growth  downwards  into 
the  coriuin ;  and  within  the  mass  of 
cells  so  deposited  the  form  of  the  hair 
makes  its  appearance  with  a  slender 
shaft  and  a  large  bulb,  into  which  a 
papilla  from  the  corium  projects.  Then, 
in  the  process  of  growth,  the  young  hair  bursts  through  the 
cuticular  sheath  in  which  it  has  been  enveloped,  and  projects 
on  the  surface. 


Fig.  42. — DEVELOP- 
MENT OF  A  HAIE,.  A, 
Downward  growth  of 
epidermis.  B,  Form 
of  the  hair  completed 
before  appearing  on  the 
surface.  After  Kolli- 
ker. 


CHAPTER  VI. 

ALIMENTATION. 

49.  IT  lias  been  already  mentioned  that  the  tissues  of  the 
body  are  constantly  parting  with  particles  which  enter  into 
their  composition,  and  are  receiving  new  materials  to  replace 
Avhat  is  lost.  We  have  seen  that  increased  muscular  exertion 
is  accompanied  with  increased  loss  of  substance;  and  the 
same  is  true  of  increased  mental  exertion,  and  of  increased 
evolution  of  animal  heat,  as  in  exposure  to  extreme  cold. 
Muscular  and  mental  effort,  and  the  maintenance  of  the 
temperature  of  the  body,  as  well  as  regeneration  of  the 
tissues,  all  involve  losses  of  substance  which  require  to  be 
made  good;  and  thus,  as  has  been  said  at  the  outset,  the  body 
may  be  regarded  as  a  vortex  whose  component  particles  are 
ever  changing  while  its  form  remains. 

The  processes  by  which  materials  are  altered  and  thrown 
off  are  all  processes  of  oxidation,  by  which  complex  chemical 
combinations  are  reduced  to  others  of  simpler  descriptions ; 
and  this  is  sometimes  expressed,  though  not  very  accurately, 
by  using  the  word  combustion  in  speaking  of  such  changes. 
The  ultimate  products  of  combustion  of  organic  matter  are, 
as  will  be  recollected,  carbonic  acid,  water,  and  ammonia; 
that  is  to  say,  that,  keeping  sulphur,  phosphorus,  and 
mineral  matters  out  of  account,  and  confining  our  attention 
to  the  carbon,  hydrogen,  oxygen,  and  nitrogen  of  organic 
matters,  the  most  complete  oxidation  of  suck  substances 
which  can  be  effected  is  into  carbonic  acid,  water,  and 
ammonia.  The  oxidation  within  the  body  falls  short  of  this; 
by  far  the  greater  part  of  the  matters  which  escape  from 
it,  after  having  circulated  in  the  system,  being  eliminated 
as  carbonic  acid,  water,  and  urea.  No  doubt,  as  has  already 
been  pointed  out,  there  is  a  continual  casting  off  of  horny 


ALIMENTATION.  77 

matters  and  of  oil  from  the  integument;  and  there  is  dis- 
charge of  undecomposed  matter  in  the  form  of  mucus  from 
intestinal  and  other  passages,  as  well  as  certain  substances 
derived  from  the  bile;  but  the  total  amount  of  loss  from 
such  sources  is  comparatively  small.  Also,  it  is  to  be  noticed, 
that  ammonia  escapes  from  the  body  in  minute  quantities, 
and  that  there  is  usually  in  the  urine  a  distinct  though  small 
quantity  of  uric  acid,  a  product  of  less  complete  oxidation 
of  nitrogenous  substance  than  urea.  But  urea,  the  principal 
organic  constituent  of  the  urine,  a  material  of  the  same 
composition  as  cyanate  of  ammonia,  and  therefore  one  stage 
removed  from  that  perfect  oxidation  which  results  in  carbonic 
acid  and  ammonia,  is  the  substance  in  the  form  of  which  by 
far  the  greater  part  of  the  nitrogenous  debris  escapes  from 
the  body. 

Keeping  out  of  consideration  the  debris  of  food  which 
has  never  entered  the  system,  but  is  discharged  in  the  feeces, 
the  amount  of  nitrogen  given  off  daily  may  be  estimated  at 
250  grains,''4  and  the  amount  of  carbon  which  escapes  in  the 
form  of  carbonic  acid  may  be  reckoned  at  4000  grains;  these 
substances  must  therefore  be  daily  introduced  into  the  system 
in  those  quantities  in  the  shape  of  food,  if  the  weight  of  the 
body  and  its  constitution  are  to  be  maintained.  Supposing 
the  250  grains  of  nitrogen  to  enter  the  system  in  the  form 
of  albumen,  then  in  consequence  of  urea  containing  a  much 
larger  percentage  of  nitrogen  and  smaller  percentage  of 
carbon  in  its  composition  than  albumen,  there  will  be  liberated 
in  the  conversion  of  the  albumen  into  urea  755  grains  of 
carbon,  or  seven-eighths  of  the  total  amount  which  the  albu- 
men contains,  to  escape  in  the  form  of  carbonic  acid,  while 
the  rest  of  the  4000  grains  daily  lost  must  be  furnished  by 
additional  supplies  of  food,  which  need  not  contain  any  nitro- 
gen. The  amount  of  solid  food  necessary  for  the  preserva- 
tion of  health  is  thus  regulated  by  the  loss  of  substance  from 
the  body;  but  as  a  certain  quantity  of  the  food  always  escapes 
digestion,  and  is  discharged,  after  traversing  the  alimentary 
canal,  without  having  entered  the  system,  the  supply  taken 
has  to  be  in  excess  of  what  is  required  to  make  up  for 
systemic  loss.  It  has  been  calculated  that  there  are  daily 
*  One  ounce  avoirdupois  contains  437J  grains. 


78  ANIMAL   PHYSIOLOGY. 

required  for  an  adult  man  about  40  ounces  of  so-called  solid 
food,  yielding  22  or  23  ounces  when  evaporated  to  dryness, 
and  from  50  to  80  ounces  of  water. 

50.  The  aliment  required  by  the  body  consists  of  organic 
food,  salts,  and  water.  The  reason  why  water  is  required  in. 
larger  proportion  in  our  aliment  than  that  in  which  it  exists  in 
the  tissues,  is  that,  by  processes  of  filtration  and  evaporation, 
the  body  is  constantly  losing  water  by  the  skin,  the  kidneys, 
and  the  lungs,  additional  to  what  is  produced  by  waste  of 
tissue.  Common  salt  or  chloride  of  sodium,  on  account  of  its 
great  solubility  and  the  ease  with  which  it  passes  through 
membranes,  is  also  particularly  liable  to  escape  both  by  the 
skin  and  the  kidneys,  and  is  found  in  all  the  secretions;  it 
requires  therefore  to  be  supplied  in  quantity  greater  than 
that  in  which  it  exists  in  the  requisite  amount  of  solid 
aliment.  The  results  of  experiment,  as  well  as  general 
experience,  show  that  the  addition  of  this  substance  to  the 
food  is  advantageous  to  nutrition ;  and  it  is  well  known  to 
farmers  that  it  is  relished  by  cattle,  and  improves  their  con- 
dition. Other  mineral  matters  useful  in  the  economy,  as, 
for  example,  salts  of  lime,  occur  in  solution  in  the  water 
which  we  drink,  as  well  as  in  our  solid  food. 

The  organic  matters  used  as  food,  like  those  which  are  met 
with  in  the  composition  of  the  body,  are  divisible  into  nitro- 
genous and  carbonaceous.  Nitrogenous  foods  are  of  two 
kinds,  namely,  albuminoid  or  proteid  substances,  and  gela- 
tinoids.  Albuminoids  are  obtainable  from  both  vegetable 
and  animal  sources,  though  much  more  abundantly  from  the 
latter;  in  flesh  diet  they  are  supplied  in  the  forms  of  muscle- 
fibrin  and  albumen ;  in  eggs  albumen  is  furnished,  mingled 
with  oil  in  the  yolk,  and  in  the  white  altogether  pure ;  and 
in  milk  the  albuminoid  constituent  is  casein,  distinguished 
from  albumen  by  not  coagulating  when  heated.  -  Under  the 
title  of  gelatinoids  may  be  conveniently  grouped  a  set  of 
substances  obtained  from  animal  sources,  and  including  not 
only  gelatin  and  chondrin,  but  the  tissues  which  yield  them. 
Also,  nearly  allied  to  those  substances  in  nutritive  value 
are  kreatin  and  other  flavouring  ingredients  in  the  juice  of 
meat. 

Carbonaceous  foods  are  likewise  of  two  principal 


ALIMENTATION.  79 

tlie  oils  and  tlie  carbohydrates.  Both,  consist  of  carbon, 
hydrogen,  and  oxygen,  but  the  oils  have  a  much  greater 
number  of  equivalents  in  their  chemical  formulae  than  the 
carbohydrates;  that  is  to  say,  they  consist  of  more  complex 
arrangements  of  atoms ;  they  have  likewise  a  larger  amount 
of  carbon  and  a  smaller  proportion  of  oxygen  in  their  compo- 
sition. The  carbohydrates  are  so  called  from  consisting  of 
carbon  in  conjunction  with  hydrogen  and  oxygen,  in  the 
proportion  in  which  they  are  combined  in  water;  they 
include  starch,  together  with  the  allied  substance  cellulose, 
which  forms  a  large  part  of  growing  vegetables,  and  sugar. 
Starch  is  a  highly  important  constituent  of  vegetable  diet, 
forming  the  larger  part  of  the  weight  of  flour,  and  a  much 
greater  proportion  of  the  substance  of  potatoes.  It  occurs 
in  small  granules  which  are  insoluble  in  cold  water,  but 
which  burst  when  boiled,  the  contents  of  the  starch  granule 
being  dissolved,  while  the  outer  envelope  remains  unacted 
on.  It  is  a  property  of  starch  that  in  the  presence  of  certain 
substances,  sometimes  termed  amylolytic  ferments,  it  becomes 
converted  into  grape  sugar ;  and  no  starch  is  capable  of  being 
absorbed  into  the  system  until  it  has  undergone  that  change. 
The  sugars  used  in  food  are  of  three  principal  descriptions — 
cane  sugar,  grape  sugar,  and  sugar  of  milk.  It  may  be  here 
remarked  that  milk,  the  sole  food  provided  by  nature  for  the 
infant,  consists  of  a  mixture  of  a  solution  of  proteid  sub- 
stance in  the  form  of  casein,  oil  in  the  form  of  butter,  sugar 
of  milk,  and  various  salts,  and  therefore  contains  all  the 
varieties  of  aliment  necessary  for  health. 

51.  Properly  to  nourish  the  body,  the  daily  waste  must  be 
balanced  by  daily  repair,  and  the  food  must  contain  a  suffi- 
ciency of  every  substance  required  by  the  tissues.  It  follows 
from  this,  and  has  been  proved  by  experiment,  that  no 
amount  of  carbonaceous  food  will  make  up  for  want  of  nitro- 
genous ingredients.  On  this  account,  animals  which  feed  on 
vegetables  have  to  consume  large  quantities  of  food  that  they 
may  extract  the  requisite  amount  of  nitrogenous  substance; 
and  persons  who  feed  entirely  on  potatoes,  require  to  use 
much  greater  quantities  than  they  would  require  of  other 
diet,  because  potatoes  consist  principally  of  starch,  and  have 
remarkably  little  nitrogenous  substance  in  their  composition. 


80 


ANIMAL    PHYSIOLOGY. 


Iii  fact,  it  may  be  said,  that  in  a  diet  sufficient  in  quantity, 
if  the  nitrogenous  constituents  be  too  plentiful,  unnecessary 
work  is  thrown  on  the  kidneys,  while,  if  they  be  too  scanty,  an 
unnecessary  load  is  thrown  on  the  intestines.  Moreover,  it 
has  been  found  by  experiment  that  no  amount  of  gelatinoid 
substance  will  suit  instead  of  the  albuminoids;  animals  can 
be  supported  on  lean  meat,  but  they  die  when  fed  on  jelly 
alone,  even  when  it  is  pleasantly  flavoured,  and  at  first 
relished.  It  appears,  therefore,  that  animals  have  no  power 
of  building  up  albuminoid  matter  from  simpler  chemical  sub- 
stances. They  have  no  power  of  manufacturing  organic 
matter  from  the  materials  found  in  inorganic  nature,  but 
feed  either  directly  on  the  vegetable  world,  or  on  other 
animals  which  have  fed  on  vegetables;  and  there  is  no  proof 
that  in  any  instance  they  have  the  power  of  acting  on  the 
simpler  organic  substances,  so  as  to  produce  from  them  the 
more  complex.  Further,  it  appears  from  the  researches  of 
botanists,  that  even  in  plants  the  power  of  building  organic 
matter  is  confined  to  the  green  parts.  The  statement  may 
therefore  be  ventured  on,  that  so  far  as  observation  has  yet 
proceeded,  it  would  appear  that  the  presence  of  chlorophyll  is 
as  necessary  for  the  production  of  organic  matter  in  organisms, 
as  the  presence  of  protoplasm  is  necessary  for  growth. 

With  regard  to  gelatin,  the  question  is  often  asked  how  it 
happens,  if  it  be  incapable  of  sustaining  life,  that  in  conjunc- 
tion with  other  things,  it  is  useful  as  an  article  of  diet,  and  a 
favourite  in  the  sick  room.  Perhaps  that  question  is  suffi- 
ciently answered  by  pointing  out  that  carbonaceous  substances 
are  likewise  insufficient  by  themselves  to  support  life,  and 
that  in  the  formation  of  urea  from  gelatin,  five-sixths  of  the 
carbon  is  unused,  and  therefore  combines  with  oxygen  to 
form  carbonic  acid,  as  does  the  carbon  of  carbonaceous  food; 
also,  that  gelatin  requires  little  digestion,  and  is  at  once  de- 
composed on  entering  the  circulation. 

All  carbonaceous  food  serves  the  economy  sooner  or  later 
by  undergoing  oxidation  into  carbonic  acid  and  water,  and 
thereby  supporting  the  temperature  of  the  body,  or  assisting 
its  vital  processes,  as  we  have  seen  that  it  does  in  muscular 
action.  Oil  may  be  temporarily  stored  up  in  the  shape  of 
adipose  tissue;  and  the  carbohydrates  may  be  stored  as 


ALIMENTATION".  81 

glycogen  in  the  liver,  a  viscus  which  likewise  serves  as  a 
reservoir  for  oil,  healthily  in  fishes,  and  more  or  less  patho- 
logically in  man;  but  there  is  no  proof  that  grape  sugar  can 
be  converted  into  oil,  nor  that  either  oil  or  sugar  is  changed 
into  any  more  complex  substance.  It  is  quite  possible  that 
the  fattening  effects  which  the  carbohydrates  often  have, 
may  be  produced  by  their  saving  from  oxidation  oil  which 
would  otherwise  be  consumed  in  their  stead.  Certain  it  is 
that,  in  dieting  the  sick,  the  use  of  the  carbohydrates  is  not 
found  to  be  equivalent  to  the  use  of  oils. 

14  F 


CHAPTER,  VII. 
DIGESTION. 

52.  THE  food  is  received  into  the  digestive  or  alimentary 
tube;  there  it  is  subjected  to  a  series  of  agencies  by  which  it 
is  in  greater  or  less  part  digested  or  reduced  to  a  condition  in 
which  ifc  can  be  sucked  up  by  appropriate  vessels ;  and,  while 
this  portion  is  absorbed  into  the  circulation,  the  effete  re- 
mainder passes  on  and  is  discharged.     The  digestive  tube, 
beginning  at  the  mouth,  is  continued  to  the  stomach  by  the 
throat  and  gullet,   while  the  stomach  is  succeeded  by  the 
small  and  great  intestine.     In  its  passage  along  this  tract, 
the  food  is  subjected  to  both  mechanical  and  chemical  pro- 
cesses; and  it  is  proposed  to  follow  its  course,  and  mark  the 
mechanical  actions  to  which  it  is  subjected,  before  directing 
attention  to  the  chemical  changes.     But,  among  the  first  of 
these  mechanical  actions  is  that  of  the  teeth;  and  the  whole 
structure  and  history  of  these  may  be  conveniently  considered 
at  the  outset,  previous  to  noticing  their  action  in  mastication. 

53.  The   Teeth. — Under  this  term,  in  its  widest  significa- 
tion, may  be  included  all  hard  structures  for  the  trituration 
of  the  food;  and  these  are  of  many  different  kinds,  and  found 
in  different  positions.     In  certain  molluscs,  e.g.,  the  genus 
Bulla,  and  in  Crustacea,  e.g.,  the  lobster  and  crab,  teeth  of 
shell  are  developed  in  the  walls  of  the  stomach ;  and  both  in 
Crustacea  and  insects,  modified  limbs,  called  jaws,  are  used 
for  the  seizure  of  food :  in  the  cuttle  fishes,  the  same  function 
is  performed  by  a  horny  beak  in  two   pa£fcs  like  that   of  a 
bird;  and  in  other  molluscs  by  the  rasping  action  of  minute 
plates  set  on  a  long  muscular  tongue.     In  the  vertebrata,  the 
same  variety  of  structures  for  breaking  down  the  food  exists. 
Birds  and  turtles  have  beaks  which  are  horny  developments 
of   the   integument  covering  the  jaws,  and  granimiverous 


THE   TEETH. 


83 


birds  have  not  only  muscular  giz- 
zards, but  by  swallowing  stones, 
furnish  themselves  with  temporary 
stomachic  teeth  of  a  most  efficient 
description. 

Even  among  teeth,  properly  so 
called,  namely,  structures  impreg- 
nated with  mineral  matter,  there  is 
great  variety.  In.  fishes  and  reptiles, 
they  may  be  jointed  to  bones  by 
means  of  ligament,  or  welded  to  them 
immovably,  and  may  be  attached  to 
numerous  bones  abutting  on  the  oral 
cavity;  indeed,  true  teeth  occur 
(Synodontis)  lying  in  the  integument 
unconnected  with  any  bone.  In  cro- 
codiles, the  teeth  are  in  large  sockets. 
In  mammals,  they  are  confined  to 
the  inferior,  superior,  and  inter-max- 
illary bones,  and  are  fastened  in 
closely  fitting  sockets,  so  that,  when 
full  grown,  they  cannot  drop  out, 
even  when  the  bones  have  been, 
macerated. 

A  tooth,  such  as  may  be  obtained 
from  the  human  subject,  consists  of 
a  crown  projecting  above  the  gum, 
and  a  roob  imbedded  in  a  socket  of 
the  jaw,  and  the  place  where  these 
meet  is  called  the  neck.  The  root 
may  consist  of  one  fang  or  several ; 
and  at  the  extremity  of  each  fang 
is  a  little  opening  leading  into  a 
cavity  in  the  interior  of  the  tooth, 
which  is  filled  with  a  soft  and 
sensitive  substance  called  the  pulp, 
while  the  blood-vessels  and  nerve  of 
the  pulp  pass  through  the  little 
opening.  The  main  mass  of  the 
hard  substance  of  the  tooth  is  COKL- 


Fig.  43. — INCISOTI  TOOTH, 
vertical  section,  a,  pulp 
cavity ;  b,  6,  dentine 
showing  the  general 
curves  of  the  tubes  and 
three  contour  lines  cross- 
ing them;  c,  enamel,  with 
coloured  bands  crossing 
its  prisms;  d,  crusta  pe- 
trosa. 


84: 


ANIMAL   PHYSIOLOGY. 


posed  of  a  structure  called  dentine,  and  this  is  covered  in 
the  crown  with  a  cap  of  enamel,  and  in  the  root  with  crusta 
petrosa. 

54.  Dentine  has  a  matrix 
yielding  gelatin,  and  impreg- 
nated with  mineral  matter  in 
slightly  greater  proportion  than 
bone;  but,  instead  of  lacunae, 
it  contains  a  multitude  of 
closely  set  tubes,  which  radiate 
from  the  pulp-cavity  in  an 
undulating  nearly  parallel 
course,  getting  smaller  as  they 
approach  the  surface,  and  giv- 
ing off  branches.  Each  tube 
contains  an  albuminoid  fibre, 
which,  at  least  in  the  young 
state,  would  appear  to  be  an 
offshoot  from  a  corpuscle  at  its 
Fig.  44. —MOLAR  TOOTH,  verti-  inner  or  pulp-extremity,  com- 
cal  section;  letters  same  as  in  parable  with  a  bone-corpuscle; 
%•  43.  and  as  the  dentine  grows  from 

without  inwards,  invading  the  pulp-cavity,  these  corpuscles 
travel  inwards  also.  Crossing  the  substance  of  the  dentine, 


Fig.  45. — DENTINE.  At  a,  the  section  is  parallel  to  the  tubes;  at  b, 
it  cuts  them  across;  c,  granular  layer  (of  Purkinje)  resulting  from 
the  presence  of  small  spaces  connected  with  the  extremities  of 
the  dentine  tubes;  d,  crusta  petrosa. 

faint  lines  may  be  often  seen,  called  contour  lines  (Owen),  which 
consist  of  chains  of  irregular  spaces  in  the  matrix,  filled  with 
less  refractive  material,  and  possibly  caused  by  the  influence 
of  the  irregular  pressure  to  which  the  teeth  are  subjected. 


f  EEM. 


85 


Enamel  is  an  exceeding  hard  substance,  containing  only 
3|-  per  cent,  of  animal  matter,  while  the  rest  of  it  consists 
of  phosphate  of  lime  and  other 
earthy  salts.  It  is  composed 
of  solid  vertical  columns  or 
prisms,  the  sides  of  which 
closely  fit  to  one  another,  like 
pillars  of  basalt,  but  are  by 
no  means  strictly  parallel,  for 
they  interlace  to  a  certain 
extent.  They  are  about  -g-^Vfr 
inch  diameter,  and  have  a 
transverse  striatioii  which  is 
particularly  distinct  in  the 
young  state.  The  enamel 
prisms  are  crossed  by  bands 
of  a  brownish  tinge  called 
coloured  lines,  which  have 
probably  a  similar  origin  to 
the  contour  lines  of  the  den- 
tine, both  being  most  fre- 
quent in  old  teeth. 


Fig.   46. — ENAMEL,      a,  a, 
tudinal   section;    &,    free 


longi- 
outer 


The  crusta  petrosa  or  cement  extremities  of  the 'prisms,  seen 
(fig.  45,  d,)  is  softer  than  in  perspective;  c,  dentine, 
dentine,  and  consists  of  a  deposit  of  layers  of  solid  bone- 
matrix,  in  which  are  sparsely  scattered  lacunae,  with  very 
irregular  canaliculi  coming  off  from  them.  In  the  human 
teeth  it  is  confined  to  the  root,  but  in  the  complex  crowned 
teeth  of  some  animals,  for  example,  the  molars  of  the  ox,  it 
fills  up  depressions  which  dip  deeply  down  into  the  crown, 
and  is  there  situated  superficial  to  the  enamel.  When  such 
a  tooth  is  worn,  as  it  soon  is  after  coming  into  use,  the  dark 
coloured  crusta  petrosa,  filling  up  the  complicated  depres- 
sions, is  seen  surrounded  by  lines  of  pure  white  enamel,  on 
the  other  side  of  which  is  the  yellow  dentine,  somewhat 
hollowed  out,  as  is  also  the  crusta  petrosa,  from  being  softer 
than  enamel. 

55.  Teeth,  in  their  first  development,  have  a  consider- 
able resemblance  to  hairs ;  for  both  make  their  first  ap- 
pearance, in  the  embryo,  from  tegumentary  depressions 


86  ANIMAL   PHYSIOLOGY. 

filled  with  epithelium,  and  with  a  papilla  at  the  bottom; 
and  in  both  the  depression  becomes  temporarily  converted 
into  a  closed  sac,  which  afterwards  is  burst  open  by  tho 
protrusion  of  the  contained  organ.  The  layer  of  epithelium 
immediately  in  contact  with  the  tooth-papilla  is  converted 
into  enamel,  the  enamel-columns  corresponding  with  the 
elongated  cells  of  the  deepset  layer  of  the  cuticle.  The 
dentine  is  formed  from  the  superficial  part  of  the  papilla 
itself,  while  the  remainder  of  that  structure  constitutes  the 
pulp.  The  development  of  the  tooth  proceeds  from  the 
summit  of  the  crown  downwards,  the  pulp  thus  becoming 
gradually  enclosed.  The  sacs  of  the  permanent  teeth  make 
their  first  appearance  from  the  necks  of  the  sacs  of  the  milk 
set  at  a  very  early  period,  while  the  latter  are  still  open,  and 
subsequently  descend  to  positions  beneath  the  milk  teeth, 
which  they  are  destined  to  replace. 


Fig.  47. — DEVELOPMENT  of  a  temporary  and  a  permanent  tooth 
(Goodsir).  a,  papilla  on  the  floor  of  the  primitive  dental  groove; 
0,  papilla  enclosed  in  a  follicle  in  the  bottom  of  the  secondary 
groove,  and  opercula  above  the  follicle;  c,  papilla  becpme  a  pulp, 
and  the  follicle  a  sac,  by  adhesion  of  the  opercular  lips,  and  tho 
secondary  groove  adherent,  except  behind  the  inner  operculum, 
where  it  has  left  a  shut  cavity  of  reserve  for  the  permanent 
tooth;  d,  e,  temporary  tooth  increasing  by  growth  downwards  of 
the  fang,  and  the  permanent  tooth-sac  receding  from  the  surface; 
/,  temporary  tooth  appearing  on  the  surface;  g,  permanent 
tooth-sac  much  removed  from  the  gum,  but  connected  with  it 
by  a  cord  which  passes  through  a  foramen  behind  the  temporary 
socket. 

56.  The  milk  teeth  are  twenty  in  number,  namely,  two 
incisors,  a  canine  or  eye  tooth,  and  two  molars,  on  each  side 
in  the  upper  and  the  lower  jaw.  The  permanent  teeth  are 
thirty-two  in  number,  namely : — incisors  and  canines,  replac- 


THE   TEETH.  87 

ing  the  corresponding  milk  teeth ;  premolars  or  bicuspids, 
replacing  the  milk-molars;  and  three  true  molar  teeth  on 
each  side  in  each  jaw,  which  are  not  preceded  by  any 
deciduous  or  milk  teeth. 


Fig.  48. — TEMPORARY  AND  PERMANENT  TEETH  in  the  jaws  of  a 
child  six  years  old.  The  temporary  teeth  are  all  still  present, 
and  the  crowns  of  the  corresponding  permanent  teeth  are  formed; 
the  first  molars  have  appeared,  and  behind  them  are  the  second 
molars  with  the  divisions  between  the  fangs  in  process  of  forma- 
tion. 

The  incisor  and  canine  teeth  are  simple,  each  having  only 
one  fang  and  one  prominence  of  the  crown;  the  crowns  of  the 
incisors  being  chisel-shaped,  while  those  of  the  canines  come 
to  a  point  or  cusp.  The  premolar  teeth  get  the  name  of  bicuspid 
because  the  crown  of  each  has  two  cusps,  an  inner  and  an  outer; 
their  fangs  likewise,  especially  those  of  the  anterior  pre- 
molars, have  a  tendency  to  divide  towards  the  extremity. 
The  molar  teeth,  both  milk-molars  and  true,  have  the  ex- 
tremity of  the  crown  thrown  into  several  elevations,  and  are 
said  to  be  multicuspid.  Those  of  the  upper  jaw  have  three 
fangs,  two  on  the  outside,  and  one  directed  inwards.  Those 
of  the  lower  jaw  have  only  two  fangs,  one  in  front  of  the 
other,  but  they  are  double  fangs,  that  is  to  say,  they  are 


£8  ANIMAL    PHYSIOLOGY. 

flattened  as  if  composed  of  two  combined,  and  have  each 
two  openings  at  the  extremity. 

57.  In  infancy  the  teeth  begin  to  cut  the  gums  about  the 
seventh  month,  the  central  incisors  appearing  first,  and  the 
others  in  order  backwards,  with  the  exception  that  the  first 
milk-molar  precedes  the  eye-tooth  in  front  of  it.     The  last 
of  this  series,  the  second  milk-molars,  appear  about  the  end 
of  the  second  year.     The  next  teeth  to  appear  are  the  first 
true  molars,  sometimes  called  the  five-year  old  teeth;  then, 
about  the  seventh  year,  the  milk  teeth  begin  to  drop  by 
absorption  of  the  fangs,  and  the  permanent  teeth  to  come 
up  above  the  gums  in  their  place.     The  changing  of  the  teeth 
begins  in  front,  and  goes  regularly  backwards,  with  the  ex- 
ception that,  as  in  the  first  dentition,  the  canines  are  delayed, 
both  bicuspids  making  their  appearance  before  the  permanent 
canines  come  up,  about  the  twelfth  year.     Soon  afterwards, 
the  second  molars  cut  the  gums,  and  lastly,  at  very  variable 
periods,  often  a  number  of  years  later,  the  third  molars,  or 
wisdom  teeth,  come  to  the  surface. 

58.  Course  of  the  Ingesta. — The  mouth,  or  buccal  cavity, 
as  it  is  technically  called  to  distinguish  it  from  the  opening 
of  the  lips,  is  walled  in  by  voluntary  muscles  of  the  face; 
within  the  arches  of  teeth  it  has  the  tongue  in  its  floor,  and 
its  roof  formed  by  the  palate,  which  separates  it  from  the 
nasal  cavity,  while  it  communicates  behind  with  the  throat, 
by  a  constricted  part  called  the  fauces.     The  arch  of  the 
fauces  is  limited  above  by  a  prolongation  backwards  of  the 
palate,  consisting  of  soft  parts  unsupported  by  bone,   and 
termed  the  soft  palate  or  velum  palati.    This  has  a  free  edge 
directed  backwards,  and  prolonged  into  a  pendulous  structure 
in  the  middle,  called  the  uvula,  while  on  each  side  descend 
from  it  two  prominences,  the  anterior  and  posterior  pillars 
of  the  fauces.     Between  these  are  placed  the  glandular-look- 
ing bodies  known  as  the  tonsils,  structures  of  obscure  func- 
tion, but  sometimes  troublesome   by    enlargement,    or   by 
inflammation  and  ulceration  in  sore  throat. 

The  cavity  behind  the  fauces  is  called  the  pharynx.  It 
is  surrounded  behind  and  on  the  sides  with  constrictor 
muscles,  while  in  front  it  communicates  with  a  number  of 
openings.  It  extends  above  the  soft  palate  to  the  base  of 


COURSE    OF   THE    INGEST  A.  89 

the  skull,  and  there  it  has  in  front  of  it  the  posterior  nares, 
or  apertures  of  the  nasal  cavities,  which  are  continued  back 
from  the  nostrils,  separated  one  from  the  other  by  a  septum. 
At  the  sides  of  the  posterior  nares  aie  the  Eustachian  tubes 
leading  back  to  the  ears.  Below  the  soft  palate,  in  series 
from  above  downwards,  are  the  arch  of  the  fauces,  the 
glottis,  or  opening  into  the  windpipe,  and  the  oesophagus, 
which  is  the  continuation  downwards  of  the  pharynx,  but, 
unlike  it,  is  a  complete  tube  with  strong  circular  and  longi- 
tudinal muscular  fibres  round  it. 


Fig.  40. — BUCCAL  CAVITY  AND  PHARYNX,  vertical  section,  a,  tongue, 
b,  nasal  or  upper  division  of  pharynx,  and  to  the  front  of  it  the 
orifice  of  the  Eustachian  tube;  c,  inferior  division  of  pharynx; 
(Z,  tonsil  with  the  anterior  and  posterior  pillars  of  the  fauces  in 
front  and  behind  it;  e,  orifice  of  duct  of  parotid  gland;  /,  the 
top  of  the  larynx,  with  the  epiglottis  in  front  of  it;  g,  oesopha- 
gus; h,  section  of  hard  palate;  s,  of  soft  palate  and  uvula. 

Loosely  connected  with  the  skull  by  ligament  and  muscle, 
is  a  slender  U-shaped  bone,  the  hyoid,  which  can  be  felt  with 
the  finder  at  the  fold  between  the  neck  and  the  chin.  Sus- 


90  ANIMAL   PHYSIOLOGY. 

pended  from  this  is  the  larynx,  or  the  upper  part  of  the 
windpipe,  modified  as  the  organ  of  voice,  while  to  its  upper 
surface  is  attached  the  root  of  the  tongue ;  and  behind  it  is 
the  epiglottis,  a  plate  of  reticular  cartilage  covered  with 
mucous  membrane,  which  in  the  state  of  rest  stands  straight 
up  in  front  of  the  glottis,  but  in  swallowing  is  folded  down 
over  that  opening,  so  as  to  prevent  anything  entering  into 
the  air  passages. 

59.  When  solid  food  is  taken  into  the  mouth,  the  teeth,  the 
tongue,  the  palate,  and  the  muscles  of  the  jaws  and  cheeks, 
combine  to  break  it  down  by  mastication.  The  jaws  act  as 
a  double  lever  of  the  third  order,  like  a  pair  of  tongs ;  the 
powerful  muscles  which  shut  them  being  placed  nearer  the 
joints  than  are  the  ranges  of  teeth  between  which  the  food 
is  crushed.  In  biting  with  the  incisor  teeth,  the  jaws  are 
simply  brought  together ;  but  in  breaking  the  food  up  with 
the  molar  ranges,  the  lower  jaw  is  slightly  rotated  from  side 
to  side,  at  the  same  time  that  it  is  withdrawn  from  and  ap- 
proached to  the  upper,  and  thus  the  food  is  rubbed  between 
the  opposed  cusps  as  well  as  bruised  by  them.  The  tongue 
presses  the  food  out  from  between  it  and  the  palate,  and 
throws  it  on  to  the  teeth  on  one  side  or  other,  thus  mixing 
it  up  and  aiding  the  thorough  permeation  of  the  saliva;  and 
the  muscles  of  the  cheeks  catch  the  portions  which  are 
crushed  outwards  by  the  teeth,  and  return  them  inwards. 

The  act  of  swallowing,  or  deglutition,  consists  of  three 
parts,  of  which  the  first  is  voluntary,  the  second  spasmodic, 
and  the  third  involuntary. 

The  purely  voluntary  part  consists  in  pushing  the  bolus 
back  between  the  tongue  and  palate  till  it  reaches  the  fauces. 
As  soon  as  it  gets  there,  the  will  initiates  a  process,  which, 
when  started,  proceeds  with  great  rapidity  and  is  finished  in 
a  moment,  without  the  will  being  able  to  delay  it.  This 
spasmodic  stage  involves  a  number  of  structures.  If  the 
finger  be  placed  on  the  hyoid  bone,  it  will  be  found  to  be 
jerked  suddenly  upwards  and  forwards;  at  the  same  time 
the  back  of  the  tongue,  on  which  the  food  has  been  placed, 
is  jerked  upwards  and  backwards;  and  any  one  who  can 
succeed  in  swallowing  with  the  mouth  open  in  front  of  a 
looking-glass,  will  see  that  the  soft  palate  is  made  tense,  and 


COURSE   OF   THE   INGESTA.  91 

that  the  two  posterior  pillars  of  the  fauces  *  for  a  moment 
approach  the  middle  line,  so  as  with  the  aid  of  the  uvula  to 
shut  off  the  nasal  part  of  the  pharynx  from  the  lower  por- 
tion. By  this  means  the  food  is  thrown  back  into  the  lower 
part  of  the  pharynx,  and  thereupon,  by  the  contraction  of 
the  pharyngeal  constrictors,  it  is  propelled  into  the  oesophagus, 
and  so  into  the  stomach.  At  the  same  time  that  provision 
is  made  to  prevent  the  food  passing  up  into  the  nose,  the 
windpipe  is  also  effectually  protected  from  the  entrance  of 
the  smallest  particle.  This  is  accomplished  by  the  upward, 
and  forward  movement  of  the  hyoid  bone,  by  which  the 
larynx  is  brought  under  cover  of  the  root  of  the  tongue, 
and  that  part  of  the  tongue,  being  also  pulled  backwards, 
pushes  with  it  the  epiglottis,  pressing  it  down  over  the 
aperture  of  the  air  passage,  while  the  contraction  of  a  few 
muscular  fibres  at  the  sides  of  that  structure  shut  it  down 
completely  like  a  lid. 

The  propulsion  of  the  food  down  the  oesophagus  is  of  the 
same  description  as  along  the  intestine.  The  oesophagus,  as 
has  been  mentioned,  is  surrounded  with  circular  and  longi- 
tudinal fibres;  the  latter  are  outermost,  as  is  the  typical 
intestinal  arrangement;  and  in  both  intestine  and  oesophagus 
what  happens  is  this : — the  longitudinal  fibres  contract  in 
the  part  of  the  tube  in  which  the  food  is,  the  circular  fibres 
contract  immediately  behind  and  over  it,  and  thus  the  food 
is  forced  on,  and,  as  it  travels,  the  wave  of  contraction  travels 
with  it.  In  the  intestine,  this  method  of  contraction  is  called 
vermicular  or  peristaltic  movement. 

60.  The  lower  end  of  the  oesophagus,  a  little  to  the  left  of 
the  middle  line,  pierces  the  diaphragm,  as  the  arched  muscular 
partition  is  called  which  separates  the  thoracic  from  the  ab- 
dominal cavity,  and  it  immediately  terminates  in  the  stomach. 

The  Stomach  is  a  large  expansion  of  the  alimentary  tube, 
which  lies  in  great  part  under  cover  of  the  lower  ribs  of  the 
left  side,  but  extends  across  the  middle  line.  It  is  of  curved 
form,  its  upper  surface  being  short  and  concave,  its  lower 
long  and  convex.  At  its  left  end,  called  also  the  cardiac 
extremity,  it  is  expanded,  and  it  gradually  becomes  narrower 

*  They  are  tlie  prominences  corresponding  to  the  edges  of  two 
muscles,  the  palato-pharyngei. 


ANIMAL   PHYSIOLOGY. 


ig.  50. — DIGESTIVE  TUBE,  a,  (Esophagus 
ending  in  the  stomach ;  6,  pylorus ;  from  b 
to  c,  duodenum;  from  c  to  d,  jejunum  and 
ileum  with  the  line  of  attachment  of  the 
mesentery;  d,  ccecum;  e,  vermiform  appen- 
dix ;  /,  g,  h,  ascending,  transverse,  and  de- 
scending colon;  i,  sigmoid  flexure;  k,  rectum. 


as  the  intestinal 
extremity  is  ap- 
proached, a  little 
to  the  right  of  the 
middle  line.  The 
entrance  into  the 
intestine  is  called 
the  pylorus,  and  is 
guarded  by  a 
strong  band  of  cir- 
cular fibres,  which 
keeps  the  opening 
usually  closed,  and 
gives  to  it  the  title 
of  pyloric  valve. 
Observations  were 
made  by  watching 
the  movements  of 
the  stem  of  a  ther- 
mometer, the  bulb 
of  which  was  intro- 
duced into  the 
stomach  of  a  man, 
through  a  fistulous 
opening  which  had 
been  left  in  the 
healing  of  a  wound 
made  by  the  acci- 
dental explosion  of 
a  gun;  and  it  ap- 
pears from  them 
that  when  the 
stomach  is  acting, 
the  contraction  of 
its  walls  causes  its 
contents  to  bo 
moved  about  in  a 
stream  passing  from 
left  to  right  along 
the  great  curvature 


COURSE    OF   THE   INGESTA.  93 

of  the  stomach,  and  back  along  the  small  curvature.  "While 
the  stomach  thus  keeps  its  contents  in  motion,  its  walls 
rapidly  absorb  superfluous  fluid,  and  at  the  same  time 
pour  out  the  gastric  juice.  By  united  mechanical  and 
chemical  action  the  food  is  gradually  converted  into  pulp; 
and  the  pyloric  valve,  which,  at  the  commencement  of  diges- 
tion, is  kept  quite  closed,  relaxes  sufficiently  to  allow  the 
pulp  to  pass  as  rapidly  as  formed,  thus  allowing  the  whole 
action  of  the  stomach  to  be  concentrated  on  the  parts  of  its 
contents  which  are  not  yet  broken  down.  Gradually,  how- 
ever, in  the  later  stages  of  digestion,  the  irritability  of  the 
pyloric  valve  becomes  exhausted,  and  at  last  even  the  most 
indigestible  solids  are  allowed  to  pass. 

61.  The  stomach  is  succeeded  by  the  small  intestine,  a  tube 
about  twenty  feet  long,  and  having  a  breadth  of  about  an 
inch  and  a  half  at  the  commencement,  and  an  inch  at  its 
termination.     For  about  the  first  ten  inches  it  is  bound  down 
in  a  crescentic  form  to  the  back  of  the  abdomen ;  and  this 
part  is  termed  the  duodenum,  and  has  the  bile  duct,  and  the 
duct  of  a  large  gland,  called  the  pancreas,  opening  into  it  close 
together  about  its  middle.     The  remainder  of  the  small  intes- 
tine is  not  connected  with  the  abdominal  wall,  except  through 
the  medium  of  the  mesentery — a  fold  of  the  peritoneum  or 
lining  membrane  of  the  abdomen,  containing  within  it  vessels 
and  nerves.     The  upper  third  of  this  part  of  the  intestine 
gets  the  name  of  jejunum  (empty),  from  usually  containing 
little  but  the  pulp  sent  down  from,  the  stomach,  and  here 
termed  chyme.     But  the  more  fluid  portions  of  the  chyme 
get  absorbed  as  it  descends,  and  in  the  lower  two-thirds  of 
the  small  intestine,  the  ileum,  the  contents  are  consequently 
usually  more  solid.     The  ileuni  opens  into  the  large  intestine 
in  the  region  of  the  right  groin. 

62.  The  large  intestine  is  about  five  or  six  feet  long.     It 
begins  in  a  short  cul-de-sac,  the  ccecum  (caput  ccecum  coli), 
below  the  entrance  of  the  ileum ;  and  into  the  extremity  of 
this  there  opens  a  small  glandular  structure,  the  vermiform 
appendage,  which  represents  the  enormously  large  intestinum 
caecum  found  in  various  animals,  such  as  sheep  and  rabbits. 
At  first  the  large   intestine  passes   directly  upwards  to  a 
point  beneath  the  liver,  and  in  this  part  of  its  course  it  is 


91 


ANIMAL   PHYSIOLOGY. 


called  the  ascending  colon  ;  it  tlien  passes  across  beneath,  the 
stomach  to  the  left  side  as  the  transverse  colon,  and  runs 
down  to  the  region  of  the  left  groin  under  the  name  of 
descending  colon ;  there  it  makes  a  loose  bend,  termed  the 
sigmoid  flexure,  and  passes  into  the  pelvis,  where  it  is  called 
the  rectum,  or  lower  bowel. 

At  the  entrance  of  the  small  intestine  into  the  great,  there 

is  an  arrangement,  called 
the  ileo-colic  valve,  to  pre- 
vent matters  which  have 
once  passed  into  the  great 
intestine  regurgitating  into 
the  ileum.  It  consists  of 
two  redundant  folds  of  the 
mucous  membrane  at  the 
opening  projecting  like  an 
upper  and  a  lower  lip  into 
the  colon,  while  from  the 
angles  of  their  junction 
prominences  of  mucous 
membrane,  the  frsena,  pass 
round  so  as  partially  to 
encircle  the  great  intestine. 
When  the  great  intestine 
is  distended,  the  frsena  are 
pulled  tight,  and  the  lips 
of  the  valve  brought  into 
Fig.  51. -ILEO-COLIC  VALVE,  a,  cut  firm  contact,  the  distending - 
end  of  the  ileum ;  b,  cut  end  of  matters  pressing  their  sur- 
the  colon;  c,  caput  csecum  coli;  d,  faces  together.  Thus,  while 
vemiifwm  appendage;  e,  lower  lip  there  is  no  0]bstacle  to  the 

passage     of    matters    from 

the  ileum  into  the  caecum,  regurgitation  backwards  is 
effectually  prevented,  and  the  more  distended  the  caecum  the 
firmer  is  the  closure  of  the  valve.  The  efficiency  of  the  valve 
is  independent  of  muscular  action,  and  can  be  exhibited 
perfectly  on  the  dead  subject. 

63.  As  might  be  expected  from  the  existence  of  the  ileo- 
colic  valve,  the  contents  of  the  intestine  on  entering  the  colon 
undergo  considerable  change  :  here  they  begin  to  acquire  a 


COURSE   OF   THE   INGESTA.  95 

foecal  odour;  yet  additional  matters  continue  to  be  absorbed 
from  them  in  their  passage  onwards.  In.  the  colon  both  the 
longitudinal  and  circular  muscular  fibres  of  the  walls  are 
collected  into  bundles;  the  longitudinal  fibres* forming  three 
bands,  and  the  circular  dividing  the  intermediate  spaces  into 
shallow  recesses;  and  thus,  although  the  total  diameter  of 
this  part  of  the  intestine  may  exceed  two  inches,  its  contents 
are  brought  well  into  contact  with  its  walls  as  they  lie  in 
these  recesses ;  and  as  they  are  pressed  onwards  from  one 
recess  to  another,  different  portions  are  brought  to  the  sur- 
face. The  rectum,  or  lower  bowel,  has  the  muscular  fibres 
regularly  disposed  around,  except  near  the  outlet,  where  the 
circular  fibres  form  a  strong  sphincter,  or  habitually  con- 
tracted ring  of  muscle,  on  which  the  power  of  retention  is 
principally  dependent,  although  it  "is  assisted  by  striped 
muscles  when  the  pressure  on  it  is  great. 

64.  The  form  of  digestive  tube,  which  has  been  briefly 
described,  is  that  which  is  found  in  the  human  subject,  but 
there  is  great  variety  in  different  animals.  In  certain  fishes, 
the  pipe  fishes  and  others,  the  tube  is  straight,  but  in  most 
of  them  it  is  convoluted ;  and  in  all  it  is  wide  from  the 
throat  to  the  pylorus,  then  constricted,  and  again  widened 
towards  the  termination;  this  latter  enlargement  being 
evidently  the  representative  of  the  large  intestine.  A  valve 
even  exists  at  the  entrance  into  the  large  intestine  in  many 
fishes.  Thus  it  will  be  seen  that  already  in  the  lowest  divi- 
sion of  vertebrate  animals,  the  digestive  tube  is  divisible  into 
three  great  parts.  In  mammals  neither  the  part  above  the 
pylorus  nor  that  below  the  ileo-colic  valve  has  a  uniform 
breadth ;  the  stomach  is  separated  from  the  pharynx  by  a 
narrow  oesophagus;  and  the  caecum  is  in  some  instances 
greatly  larger  than  the  rest  of  the  great  intestine.  The 
stomach  and  caecum  are  the  parts  which  undergo  the  most 
remarkable  complications.  The  human  stomach  is  an  instance 
of  the  simplest  and  typical  form  of  the  organ  in  mammals ; 
but  in  various  animals  belonging  to  widely  separate  orders, 
remarkable  complications  exist,  e.g.,  in  the  camel,  the  kan- 
garoo, the  cetacea,  the  peccary,  and  certain  monkeys. 

Probably  the  arrangement  most  interesting  to  the  general 
reader  is  the  ordinary  ruminant  stomach^  which  is  clividecl 


9b  ANIMAL   PHYSIOLOGY. 

into  four  compartments.  When  the  ruminant  is  feeding, 
the  food  passes  into  a  large  compartment,  the  paunch, 
which  is  the  cardiac  extremity  of  the  stomach,  elongated  and 
folded  on  itself  in  foetal  life,  and  subsequently  much  expanded. 
When  the  animal  lies  down  to  ruminate,  the  contents  of  the 
paunch  are  propelled  in  successive  portions  into  a  small  com- 
partment called  the  reticulum,  from  the  honey-combed  appear- 
ance of  its  walls,  and  placed  to  the  front  below  the  oesophagus, 
and  by  it  are  returned  to  the  mouth  in  separate  pellets. 
After  being  a  second  time  masticated,  the  food  is  again 
swallowed,  and  being  prevented  by  the  closure  of  a  ring  of 

muscular  fibres  from  re-entering 
the  first  two  compartments,  it 
falls  into  the  third,  the  omasum, 
which  lies  behind  the  second, 
and  is  of  similar  size,  but  has 
its  mucous  membrane  thrown 
into  numerous  broad  folds 
which  separate  up  the  bolus, 
and  allow  it  to  pass  in  portions 
Fig.  52.— RUMINANT  STOMACH,  gradually  into  the  fourth  com- 
cliagrammatic  view.  partment  or  abomasum,  which 

is  the  pyloric  end  of  the  stomach,  and  the  only  part  furnished 
with  a  highly  vascular  and  glandular  mucous  membrane. . 

The  coecum  may  acquire  a  great  development  even  in 
animals  in  which  the  stomach  is  simple.  This  is  the  case 
in  the  rabbit  and  in  the  horse,  in  both  which  it  has  great 
width  and  length.  Generally,  the  alimentary  tube  is  much 
more  elongated  and  complex  in  vegetable-feeders  than  in 
carnivora. 

65.  Digestive  Fluids. — The  first  substance  added  to  the 
food  is  the  saliva,  which  is  furnished  by  three  pairs  of 
compound  sacculated  or  lobulated  glands — the  p.arotid,  the 
submaxillary,  and  the  sublingual.  The  parotid  gland  is 
the  largest,  and  is  named  from  its  nearness  to  the  ear,  lying 
as  it  does  in  the  hollow  between  the  ear  and  the  lower 
jaw.  It  turns  over  the  hinder  border  of  the  jaw;  and 
from  this  part  of  it  issues  a  duct — the  duct  of  Stenson — large 
enough  to  admit  a  crow-quill,  and  opening  into  the  mouth 
through  the  cheek,  opposite  the  second  molar  tooth  of  the 


DIGESTIVE   FLUIDS. 


97 


tipper  jaw.     The  submaxillary  gland,  about  the  size  of  a 
large  prune,  lies  beneath  the  lower  jaw,  and  its  duct — the 
duct  of  Wharton — opens  under  the  tongue.    The  termination 
of  this  duct,  along  with  its  fellow,  may  be  seen  in  a  looking- 
glass,   making  a   little  swelling   on  the  lower  part  of  the 
frsenum  or  bridle  of  the  tongue.     At  the  same  time  will  be 
seen  on  each  side  of  the  frsenum,  between  the  tongue  and 
the  jaw,   an  eleva- 
tion about  the  size  \ 
of  an  almond,  with 
the  outlines  of  the 
sublingual       gland 
seen     through    the 
mucous  membrane. 
It       has       several 
ducts,       some       of 
which   open   separ- 
ately,  while  others 
fall  into  Wharton's 
duct.    Besides  these 
large  glands,  there 
are  numerous  others 
of  small  size  which 
open       into       the 
mouth,  particularly 

the    buccal  glands,  Fig.  53.— SALIVARY  GLANDS,     a,  parotid;  6, 
a    series     of    struc-  submaxillary;  c,  sublingual. 

tures  the  size  of  lentils,  scattered  under  the  mucous 
membrane  of  the  lips  and  cheeks,  and  a  smaller  set  of  glands 
near  the  tip  of  the  tongue;  but  they  are  engaged  in  the 
secretion  of  a  mucus  which  has  little  or  nothing  to  do  with 
digestion.  The  saliva  has  a  slightly  alkaline  reaction,  and 
its  most  important  ingredient,  besides  water,  is  a  nitrogenous 
substance  called  ptyalin  which  has  in  a  very  high  degree  the 
property  of  converting  starch  into  grape-sugar,  provided  that 
the  solution  be  alkaline.  So  powerful  is  this  action,  that  a 
notable  amount  of  the  starch  taken  into  the  mouth  as  food 
is  converted  into  dextrin  or  sugar  before  being  swallowed; 
and  when  the  food  is  of  a  tenacious  consistence,  masticated 
and  taken  in  quantity,  a  large  part  of  it  may  escape  for  a 
14  G 


98  ANIMAL   PHYSIOLOGY. 

time  the  contact  of  the  acid  gastric  juice,  and  have  a  much 
larger  quantity  of  its  starch  acted  on  in  the  stomach.  The 
conversion  of  starch  into  sugar,  however,  is  neither  the  only 
nor  even  the  principal  use  of  the  saliva,  as  is  proved,  first, 
from  the  salivary  glands  being  well  developed  in  carnivorous 
animals,  which,  in  the  natural  state,  use  no  starch  in  their 
food;  and,  secondly,  from  the  amount  of  saliva  mixed  with 
the  food  bearing  a  direct  proportion  to  its  dryness — a  circum- 
stance which  shows  that  it  is  important  as  a  moistener. 

66.  In  the  stomach  the  food  causes  a  flow  of  gastric  juice, 
an  acid  secretion  with  a  peculiar  nitrogenous  principle,  called 
pepsin.     Various  acids  may  be  developed  among  the  contents 
of  the  stomach,  the  most  important  of  which  are   lactic, 
acetic,  and  butyric ;  but  these  are  generated  accidentally  in 
the  processes  of  change,  and  it  has  been  shown  by  investiga- 
tions, both   on  the  lower  animals  and  on  man,  that  free 
hydrochloric  acid  is  that  which  is  secreted  by  the  glands  of 
the  stomach.     The  use  of  the  acid,  however,  appears  to  be 
to  aid  the  pepsin ;  the  peculiar  properties  of  that  substance 
being  exhibited  in  none  but  acid  solutions  :  and  for  this 
purpose  any  acid  suffices.     Pepsin  is  obtainable  from  the 
mucous  membrane  of  the  stomach  of  animals  by  the  action 
of  water.     It  is  the  active  principle  in  the  rennet  iised  in 
manufacturing  cheese;    rennet  being  a  preparation  of  the 
mucous  membrane  of  a  calf's  stomach,  used  on  account  of  the 
property,  which  pepsin  possesses,  of  curdling  casein.     In  an 
acid  solution,  at  a  temperature  of  about  100°.F.,  pepsin  dis- 
solves coagulated  albumen,  and  renders  it  incapable  of  being 
coagulated  again  by  heat.     It  acts  in  like  manner  on  nearly 
all  albuminoids,  and  the  substances  into  which  it  converts 
them  are  called  peptones;  but  the  chemical  nature  of  these 
is  not  thoroughly  understood.     It  likewise  dissolves  gelatin 
and  gelatiniferous  tissue,  converting  them  into  a  solution 
which  does  not  jelly  on  cooling ;  but  it  has  no  action  on 
either  oil  or  starch.    The  gastric  juice  therefore  digests  none 
but  nitrogenous  substances. 

67.  The  mucous  membrane  of  the  stomach,  by  which  the 
gastric  juice  is  secreted,  is  thick,  soft,  and  smooth.     Looked 
at  under  the  microscope,  the  surface  is  seen  to  be  thrown, 
into  shallow  pits,  into  each  of  which  open  several  small 


DIGESTIVE   FLUIDS. 


99 


glands;  and  in  sections  vertical  to  the  surface  the  mucous 
membrane  is  shown  to  consist  in  greater  part  of  tubular 
glands,  sometimes  branched,  called  the  gastric  follicles,  which 
may  reach  -^  inch  in  length,  and  are  crowded  together. 
They  are  all  lined  with  columnar  epithelium  down  to  the 
bottom,  and  the  epithelial  cells  have  been  seen  to  undergo 
a  change  of  appearance 
in  digestion,  when  the 
gastric  juice  is  secreted ; 
but  the  majority  of  the 
follicles  likewise  contain 
numbers  of  large  oval 
and  more  easily  dis- 
covered cells  underneath 
the  columnar  epithelium, 
structures  of  uncertain 
function,  which,  from 
having  been  supposed  to 
be  the  agents  in  secret- 
ing pepsin,  have  been 
named  peptic  cells. 

The  glands  in  which 
these  cells  occur  are 
called  peptic  glands, 
while  the  others  are 
termed  mucous.  In  some  animals,  for  example  the  dog, 
certain  regions  of  the  stomach  contain  none  but  peptic 
glands,  while  the  pyloric  region  has  none  but  mucous  glands; 
and  this  circumstance  has  been  taken  advantage  of  to  deter- 
mine the  character  of  the  secretion  furnished  by  the  two 
kinds  of  glands.  But  while  one  observer  (Ebstein)  gives  a 
most  careful  and  convincing  account  of  experiments  to  prove 
that  both  kinds  secrete  pepsin,  numerous  others  come  to  a 
different  conclusion ;  and  it  may  be  considered  as  still  un- 
decided whether  the  peptic  or  the  columnar  cells  are  the 
agents  by  which  pepsin  is  really  secreted. 

68.  Immediately  beyond  the  pyloric  valve,  on  entering  the 
intestine,  the  mucous  membrane  changes  its  character.  Not 
only  is  it  firmer  and  thinner,  but  it  is  covered  over  with 
minute  thread-like  projections  like  velvet  pile,  named  mlli, 


Fig.  54. — GASTRIC  "FOLLICLES.  A,  Ver- 
tical section  of  mucous  membrane 
from  the  middle  of  the  human  stom- 
ach. B,  A  more  highly  magnified 
view  of  portion  of  a  gastric  follicle 
of  a  dog,  showing  both  peptic  and 
columnar  cells.  From  Heidenhain. 


100 


ANIMAL   PHYSIOLOGY. 


best  brought  into  view  by  laying  the  opened  intestine  in 
water,  so  as  to  make  them  float.  They  are  as  much  as  half 
a  line  long  in  the  duodenum,  and  extremely  crowded,  but 
gradually  get  shorter  and  fewer  in  the  lower  parts  of  the 

small    intestine,     until, 
near  the  termination  of 
the  ileum,  they  degene- 
rate into  scattered  wart- 
like  eminences.  The  villi, 
are  confined  entirely  to 
the  small  intestine,  and 
are  a  means  of  increas- 
ing the  extent  of  mucous 
membrane  for  the  pur- 
pose of  absorption ;  each 
villus  being  dipped  like 
a  finger  in  the  chyme, 
and  sucking  up  the  di- 
Fig.   55.  —  VALVUX.E     CONNIVENTES,    gested    parts    of    it    by 
exhibited  in  a  portion  of  jejunum    every  portion  of  its  sur- 
cut  open.     Two-thirds  natural  size.       face<        The      absorbent 
surface   of    the    small    intestine   is    further    increased    by 
crescentic  folds  of  the  mucous  membrane  passing  more  than 
half  round   it,   and  dipping   into  the  interior.      They  are 
called  valvulce    conniventes,    and,    like    the  villi,  are    most 
numerous    and     well    developed    in    the    duodenum     and 
jejunum. 

The  thickness  of  the  mucous  membrane  of  the  small  intes- 
tine is  occupied,  like  that  of  the  stomach,  with  closely  set 
tubules,  only  they  are  much  smaller  than  those  of  the 
stomach,  contain  no  other  than  columnar  epithelium,  and 
are  quite  simple ;  these  are  called  the  follicles  or  crypts  of 
Lieberkuhn.  The  duodenum  has  likewise  three  other  secre- 
tions poured  into  it,  namely,  the  bile,  the  pancreatic  juice, 
and  the  secretion  of  a  number  of  minute  glands  of  a  lobu- 
lated  description,  scattered  beneath  its  mucous  membrane, 
and  called  Brunner's  glands. 

69.  The  pancreas  is  a  large  lobulated  gland,  about  eight 
inches  long  and  one  inch  and  a  half  broad,  not  unlike  the 
salivary  glands  in  appearance,  and  sometimes  called  by 


DIGESTIVE   FLUIDS. 


101 


the  Germans  the  abdominal  salivary  gland;  its  secretion, 
however,  is  much  more  important.  The  pancreatic  juice, 
which  is  a  viscid  alkaline  fluid  containing  an  albuminoid 
principle,  has  a  most  powerful 
action  in  converting  starch  into 
sugar,  and  has  the  advantage  over 
saliva,  that  this  action  is  not  pre- 
vented by  the  presence  of  acid.  It 
has  also  the  property,  at  the  tem- 
perature of  the  body,  of  making  a 
very  complete  emulsion,  or  milky 
fluid,  with  oils;  that  is  to  say,  it 
resolves  them  into  exceedingly  min- 
ute globules,  which  remain  separate 
from  one  another ;  and  these  find 
their  way  through  the  walls  of  the 
villi  into  the  absorbent  vessels.  In 
rabbits  the  duct  of  the  pancreas 
opens  into  the  intestine  considerably 
lower  than  the  bile  duct ;  and  in 
them,  as  Bernard  has  observed, 
there  are  no  oil  globules  in  the  ab- 
sorbent vessels  above  the  level  of 
the  pancreatic  duct;  which  shows  Fig. 
that  the  bile  is  incapable  of  digest- 
ing fats  without  the  pancreatic  juice. 
This  fluid  has  likewise  been  shown 
to  have  a  solvent  action  on  albumi- 
noids outside  the  body ;  and  it  is 


56.  —  DUODENUM, 
vertical  section  magni- 
fied, a,  Villi;  &,  Lieber- 
kiihnian  follicles ;  c, 
Briinner's  gland  in  the 
submucous  tissue ;  d, 
muscular  walls. 


possible,  as  has  been  suggested  by  one  observer  (Flint) 
that  muscular  fibre,  disintegrated  by  the  gastric  juice,  is 
afterwards  completely  dissolved  by  means  of  the  pancreatic. 
It  appears,  then,  that  the  pancreatic  juice  is  the  principal 
means  of  digesting  starch  and  oil,  and  that  it  is  likewise 
useful  in  digesting  albuminoids. 

70.  The  action  of  the  fluid  secreted  by  the  Lieberkuhnian 
follicles  (succus  inteslinalis),  has  been  examined  by  gathering 
it  from  the  intestines  of  animals  experimented  on,  and  by 
other  means.  Like  the  pancreatic  juice,  it  is  an  alkaline 
fluid  which  acts  on  starch,  oil,  and  albuminoid  matters;  but 


102 


ANIMAL   PHYSIOLOGY. 


its  action  is  -weak  compared  with  that  of  the  gastric  and 
pancreatic  juices. 


Fig.  57. — DUODENUM,  PANCREAS,  LIVER,  AND  SPLEEN.  "  'a,  Duo- 
denum ;  6,  pancreas,  with  the  pancreatic  duct  laid  bare ;  c,  d, 
right  and  left  lobes  of  the  liver,  seen  from  below ;  e,  obliterated 
umbilical  vein ;  /,  gall  bladder,  opening  into  the  cystic  duct 
which  joins  with  the  hepatic  duct,  to  form  the  ductus  communis 
choledochus;  g,  spleen. 

71.  In  considering  the  use  of  the  bile  in  the  intestine,  it  is 
necessary  to  note  that  the  liver,  as  will  be  shown  in  another 
place,  is  not  a  mere  preparer  of  a  digestive  fluid,  and  that 
bile  seems  rather  to  be  formed  in  connection  with  a  blood- 
purifying  process  in  the  liver,  than  as  an  aid  to  digestion. 
The  bile  is,  however,  undoubtedly  of  great  service  in  tho 
absorption  of  oils.  In  dogs  in  which  the  gall  ducts  have 
been  shut  off  from  the  intestine,  and  the  bile  allowed  to 
escape  by  an  external  opening,  the  operation  is  followed  by 
greatly  impaired  power  of  absorbing  oils.  Indeed,  bile  is 
the  strongest  soap  known,  ox-gall  being  on  that  account  used 
by  painters  to  wash  away  oil  paint;  and  it  seems  probable 
that,  by  lubricating  the  villi,  the  bile  assists  the  minute 
globules  in  the  emulsion  formed  by  the  pancreatic  juice  to 
permeate  the  mucous  membrane.  Bile  is  likewise  an  anti- 
septic, and  prevents  the  putrefaction  in  the  contents  of  the 
intestine,  which  always  takes  place  when  its  flow  is  pre- 
vented. But,  in  addition  to  all  this,  it  is  to  be  observed 


DIGESTIVE    FLUIDS. 


103 


that,  in  health,  the  essential  constituents  of  the  bile  are  not 
to  be  found  in  the  faeces,  only  the  colouring  matter  is  left; 
also,  dogs  from  which  the  bile  is  abstracted  by  a  fistulous 
opening,  are  remarkably  ravenous.  These  facts  seem  to 
point  out  that  the  bile,  which  acts  as  a  narcotic  poison  when 
introduced  unchanged  into  the  blood,  is  decomposed  in  the 
intestine,  and  gives  rise  to  innocuous  products,  which  are  re- 
absorbed  to  nourish  the  body.  The  seat  of  this  decomposi- 
tion appears  to  be  the  great  intestine  ;  for  there  the  bile  dis- 
appears, and  various  substances  are  found  which  are  obviously 
derived  from  its  constituents.  Of  these,  two  are  crystalline, 
viz.,  excretinj  a  carbonaceous  substance  containing  sulphur 
(Marcet),  and  stercorin,  a  body  derived  from  cholesterin 
(Flint). 

72.  The  great  intestine  has  a  smooth  mucous  membrane, 
with  tubular  follicles  like  those  of  the  small  intestine,  but 
apparently  having  a  different  secretion.  The  details  of  the 
processes  which  take  place  in  this  part  of  the  alimentary 
tube  are  but  little  understood.  The  bulk  of  the  faeces 
consists  of  matters  which  have  resisted  all  digestive  pro- 
cesses, and  always  contains  starch  grains,  muscular  fibre,  and 
vegetable  tissues,  when  these  substances  have  formed  part  of 
the  diet. 


.  58. — SECTION  OF  AN  INJECTED  TONSIL,    a,  a,  Mucous  membrane 
of  fauces;  6,  a  recess;  c,  c,  c,  closed  follicles. 

73.  Before  leaving  the  consideration  of  the  alimentary  tube, 


104 


ANIMAL 


a  set  of  structures  of  obscure  function,  found  in  every  part 
of  it,  may  be  mentioned,  namely,  the  closed  follicles.  These 
are  bodies  the  size  of  small  millet  grains,  imbedded  in  the 
mucous  membrane,  and  containing  corpuscular  matter  and 
loops  of  capillary  blood-vessels.  A  number  of  such  struc- 
tures, grouped  round  recesses  of  the  mucous  membrane  of 
the  fauces,  constitute  the  tonsils  (p.  88) ;  they  are  also 

numerously  scattered  in  the 
pharynx,  and  are  called  lenti- 
cular glands  in  the  stomach. 
In  the  ileum  they  are  gathered 
in  elongated  and  rounded 
groups  about  half  an  inch 
broad,  called  Peyer's  patches, 
or  agminated  glands;  and  in 
the  great  intestine  they  are 
very  plentifully  scattered  all 
over,  and  called  solitary  glands. 
In  recent  years  they  have  been 
very  generally  supposed  to  be 
connected  with  the  absorbent 
system;  but  there  is  no  suffi- 
cient proof  that  their  function 
is  not  secretory,  although  their 
Fig.  59.— Two  PEYER'S  PATCHES,  structure  differs  from  that  of 
natural  size.  secreting  glands.  Peyer's 

patches  are  interesting  as  being  the  seat  of  a  deposit  in 
typhoid  fever,  and  subject  to  ulceration  in  that  disease. 


CHAPTER  VIII. 
THE  BLOOD. 

74.  HAVING  traced  the  process  of  digestion,  it  would  be 
natural  to  pursue  the  history  of  the  new  supplies  of  nourish- 
ment after  their  entrance  into  the  economy  from  the  alimentary 
tube.     It  will  be  found,  however,  to  be  more  convenient,  if, 
instead  of  adhering  strictly  to  the  course   taken  by  these 
supplies,   we  first  consider   the  blood,   and  afterwards  the 
streams  which  fall  into  it,  and  its  mode  of  elaboration. 

On  account  of  the  extreme  facility  with  which  substances 
pass  outwards  and  inwards  between  the  minutest  vessels 
and  the  tissues,  and  the  impossibility  of  completely  emptying 
the  vascular  system,  even  in  the  bodies  of  animals,  it  is 
exceedingly  difficult  to  estimate  the  amount  of  the  blood. 
In  one  observation,  in  which  the  blood  was  carefully  washed 
from  the  bodies  of  two  executed  criminals,  and  the  calcula- 
tion based  on  the  amount  of  solid  matter  obtained,  the 
weight  of  blood  was  estimated  as  one-eighth  of  that  of  the 
body  (Weber  and  Lehmarm).  According  to  other  calcula- 
tions founded  on  observations  011  animals,  and  made  by 
mixing  a  portion  of  blood  with  a  known  amount  of  water, 
then  washing  out  the  vessels,  and  reducing  the  washings  to 
the  same  tint  as  the  standard  solution,  it  was  computed  at 
about  one-thirteenth  of  the  weight  of  the  body,  or  twelve 
pounds  in  a  person  eleven  stones  weight  (Welcker). 

75.  When  blood  flows  from  a  wound  it  speedily  coagulates 
or  runs  into  a  clot.     This  depends   on  the  presence  of  .a 
spontaneously  coagulable  albuminoid  substance,  called  fibrin, 
which,  being  diffused  through  the  blood,  entangles  the  other 
constituents  in  its  meshes.     But  if  the  blood  be  allowed  to 
remain  in  a  vessel,  the  coagulum  contracts,  and  expels  from 
it  the  serum,  a  straw-coloured  fluid  which  may  be  more  or 


106  ANIMAL   PHYSIOLOGY. 

less  tinged  with  red.  If,  while  the  blood  is  flowing,  in  bleed- 
ing from  the  arm,  the  physician  whips  or  rapidly  stirs  it  with 
a  bunch  of  small  rods,  as  sometimes  used  to  be  done,  the  fibrin 
will  adhere  in  tough  coagulated  masses  to  the  rods,  and  the 
remainder  of  the  blood  will  remain  fluid.  This  defibrinated 
blood,  after  a  while,  will  separate  into  clear  serum  above,  and 
a  dark  dense  portion  below.  An  examination  of  blood  under 
the  microscope  shows  it  to  consist  of  a  clear  fluid  with  a 
multitude  of  coloured  corpuscles  floating  in  it;  and  the 
separation  of  the  defibrinated  blood  into  two  parts,  is  the 
result  of  these  corpuscles  falling  to  the  bottom  of  the  liquid 
serum.  If  blood  be  taken  from  a  person  in  certain,  exceptional 
conditions,  or  if  it  be  taken  from  a  horse,  instead  of  the 
whole  mass  becoming  converted 


corPusc^es  wiU  subside  consider- 
ably  before  coagulation  sets  in, 
and  then  there  will  be  in  the  upper 
part  of  the  vessel  a  straw-coloured 
coagulum  forming  a  transparent 

Jell7-     Tnat  is  a  state  of  Batters 
which  formerly  medical  practition- 
ers considered  as  a  certain  sign  of 
inflammation,    and   described    by 
saying  that  the  blood  was  'buffed. 
Fig.  60.—  HUMAN  BLOOD  Con-  The  clear  coagulum  in  contracting 
PUSCLES.     a,  a,   Bed  cor-  becomes    loosened  from  the  edge 


, 

puscles  of  smaller  size,  such  upper  surface,  while  a  pure  serum 
as  are  now  and  then  seen  is  pressed  out  at  the  sides  and 
exceptionally;  c,  c,  red  cor-  £nto  t^e  concavity  above  :  and  this 
LTtS1oew?f  tf^d  <*edd  practitioners  expressed  by 
corpuscles  shrivelled  by  saying  that  the  blood  was  cupped. 
partial  evaporation  of  the  These  observations  furnish  a 
serum;  e,  a  red  corpuscle  r0ngh  but  instructive  analysis  of 

jr^/SftSSS  ^  ?ree  consf  ue±  are  ex- 

puscles;  g,  one  which  has  hibited,  the  corpuscles,  the  serum, 

undergone  amoeboid  change  and  the  fibrin.      The   serum  and 

of  form.   ,^  ,       fibrin  together  constitute  the  liquor 

sanguinis,  or  blood  plasma.     The  blood  may  thus  be  separ- 


THE   BLOOD. 


107 


inch  diameter 


ated  into  liquor  sanguinis  and  corpuscles,  or  into  defibriii- 
ated  blood  and  fibrin,  or,  lastly,  into  serum  and  a  coloured 
clot  consisting  of  the  fibrin  with  the  corpuscles  entangled  in  it. 

76.  When  blood  is  examined  microscopically  it  is  seen  to 
contain  two  kinds  of  corpuscles,  the  coloured  kind  already 
alluded  to,  and  a  less  numerous  set  of  white  corpuscles. 

The  coloured  or  red  corpuscles  are,  properly  speaking,  deep 
orange,  as  may  be  seen  by  streaking  blood  on  a  white  sur- 
face. They  are  disc-shaped  bodies,  about 
in  man,  circular,  and  flat  or  somewhat 
concave  on  each  side.  They  are  clear, 
and  in  mammals  are  destitute  of  nucleus; 
but  this  is  a  mammalian  peculiarity, 
for  in  all  other  vertebrate  animals  they 
have  a  nucleus  and  are  oval.  In.  the 
camel  tribe  they  are  likewise  oval,  but 
are  destitute  of  nucleus  as  in  other 
mammals.  In  different  vertebrate  ani- 
mals the  red  corpuscles  differ  greatly 
in  size,  as  indeed  do  other  textural  ele- 
ments. Their  size  is  dependent  more 
on  the  affinities  of  the  animal  than  on 
its  bulk.  In  ruminants  generally  they 
are  small;  and  in  the  smallest  ruminant, 
the  musk  deer,  their  diameter  is  only 


Fig.  61. — BLOOD  COR- 
PUSCLES OF  FROG,  viz., 
four  red  corpuscles 
seen  in  full  view,  one 
in  profile,  and  one 
white  corpuscle. 


12a2-5  of  an  inch.  In  birds  they  are  smaller  than  in  reptiles ; 
and  those  of  greatest  size  are  found  in  the  amphibia,  the 
largest  known  being  those  of  the  proteus,  which  are  -^^  of  an 
inch  in  length. 

Hed  corpuscles  contain  a  firm  framework  or  stroma, 
besides  their  coloured  contents;  but  it  is  difficult  to  believe 
that  they  have  any  envelope,  when  one  sees  the  great  power 
of  elongation  which  they  have  in  threading  their  way  through 
narrow  passages,  and  the  changes  of  shape  which  they  under- 
go in  various  circumstances  outside  the  body,  without  ex- 
posure of  a  membrane.  They  may  often  be  seen  to  become 
indented  round  the  edges;  and  the  processes  between  the 
indentations  may  grow  to  a  length  which  seems  inconsistent 
with  the  supposition  that  they  are  firmer  toward  the  surface 
than  within. 


108  ANIMAL    PHYSIOLOGY. 

In  blood  which  has  a  tendency  to  "  buff,"  the  red  corpuscles 
are  seen  under  the  microscope  to  arrange  themselves  in 
columns  like  rows  of  coin,  their  cohesive  attraction  one  to 
another  being  increased,  or  that  between  them  and  the 
liquor  sanguinis  being  diminished.  The  main  function  of  the 
red  corpuscles  is,  as  we  shall  find,  to  carry  oxygen. 

77.  The  white  or  colourless  corpuscles,  also  termed  leucocytes, 
are  spherical  in  form,  larger  than  the  red,  being,  in  man,  about 
g-oVo"  inch  in  diameter,  or  more  than  that.  They  have  a 
turbid  or  mottled  appearance,  which,  on  addition  of  water, 
disappears  and  discovers  a  nucleus.  When  coagulation  is 
retarded,  and  the  red  corpuscles  sink,  the  white  corpuscles 
rise  gradually  to  the  top,  showing  that  they  are  lighter  than 
the  fluid  part  of  the  blood,  while  the  red  corpuscles  are 
heavier.  Watching  them  as  they  circulate  in  the  capillary 
vessels  of  the  web  of  a  frog's  foot,  one  may  see  that  the  white 
corpuscles  often  show  a  tendency  to  adhere  to  the  wall  of 
the  vessel,  while  the  red  corpuscles  keep  in  a  stream  in  the 
centre  of  it;  and  it  has  been  proved  by  repeated  observation 
that  white  corpuscles  are  capable  of  making  their  way  through 
the  capillary  wall,  which  comes  together  again  behind  them 
without  apparent  breach  of  continuity,  while  they  pass  on 
into  the  tissue.  It  would  appear  that  red  corpuscles  some- 
times pass  out  in  the  same  way;  but  the  white  have  a  much 
greater  tendency  to  escape,  and  after  they  have  done  so,  no 
line  can  be  drawn  between  them  and  the  other  amoeboid 
bodies,  that  is  to  say,  the  connective-tissue-corpuscles,  for  the 
white  corpuscles  have  amoeboid  properties.  Moreover,  pus, 
the  matter  thrown  out  in  suppuration,  consists  of  a  fluid  rich 
in  corpuscles,  which  cannot  be  separated  by  any  line  of  dis- 
tinction from  white  corpuscles ;  and  it  is  not  yet  settled  to 
what  extent  these  consist  of  transuded  white  corpuscles,  or 
how  far  they  are  derived  from  processes  of  multiplication 
among  the  connective-tissue-corpuscles.  But  whatever  may 
be  the  occasional  functions  of  the  white  corpuscles  exercised 
by  escaping  into  the  tissues,  they  seem  to  have  a  much  more 
important  purpose  within  the  circulation,  for  it  is  probable 
that  the  red  corpuscles  are  formed  from  them  by  disappear- 
ance of  the  nucleus  and  alteration  of  their  contents. 

We  shall  find  that  the  white  corpuscles  take  origin  in  tho 


THE    BLOOD.  109 

spleen  and  in  the  lymphatic  glands;  they  appear  in  great 
numbers  immediately  after  eating,  and  quickly  disappear 
again.  Thus,  a  German  observer  (Hirt)  computed  the  pro- 
portion of  white  to  red  corpuscles  in  his  own  blood,  and 
found  that  before  breakfast  it  was  1  in  1800,  an  hour  after- 
wards 1  in  700,  and  between  eleven  and  twelve  o'clock  1  in 
1500.  He  took  dinner  at  one  o'clock,  after  which  the  pro- 
portion was  1  in  400,  while  two  hours  afterwards  it  sunk  to 
1  in  1475.  After  an  eight  o'clock  supper  it  was  1  in  550, 
and  at  eleven  o'clock  1  in  1200. 

78.  Turning  now  to  the  chemical  composition  of  the  blood, 
we  find  that  the  liquor  sanguinis  is  essentially  an  albuminoid 
solution.  It  is  slightly  alkaline,  and  contains  about  97  parts 
of  solid  matter  in  every  thousand.  Of  these  only  four  parts 
consist  of  fibrin ;  while  the  albumen,  which  is  the  principal 
constituent  of  the  serum,  forms  nearly  79  parts ;  the  mineral 
matters  constitute  more  than  8  parts ;  urea,  kreatin,  and 
other  matters  soluble  in  water,  usually  grouped  together 
under  the  name  of  extractive,  make  up  about  4,  and  the  fats 
less  than  2  parts  in  the  thousand. 

The  blood  is,  in  health,  very  uniform  in  its  composition, 
and  it  will  naturally  occur  to  ask  how  the  uniformity  is 
maintained,  seeing  that  the  additions  made  to  it  must  vary 
much  with  the  character  of  the  diet.  It  may  also  be  asked 
how  it  happens  that  the  blood,  which  nourishes  the  whole 
body,  has  so  little  resemblance  to  the  total  composition  of  the 
body.  Both  these  questions  admit  of  one  answer,  namely, 
that  the  amount  of  any  substance  in  the  blood  at  one  time 
depends  not  only  on  the  quantity  which  enters  the  circulation, 
but  on  the  length  of  time  that  it  remains  there.  Thus,  there 
is  very  little  fatty  matter  in  the  blood,  although  quantities 
enter  with  the  supplies  of  nourishment  from  the  food,  and  at 
least  one  substance,  roughly  classed  under  this  head,  choles- 
terine,  is  returned  from  the  brain  ;  and  there  is  only  a  very 
minute  quantity  of  urea  in  the  blood,  although  there  is 
reason  to  believe  that  a  considerable  amount  of  what  is 
eliminated  by  the  kidneys  pre-exists  in  it :  but  the  explana- 
tion is  simply  that  none  of  these  substances  are  allowed  to 
accumulate  in  the  blood,  that  they  are  removed  from  it  as 
speedily  as  they  enter  it. 


110  ANIMAL   PHYSIOLOGY. 

79.  The  small  amount  of  fibrin  in  the  liquor  sanguinis,  com- 
pared with  the  quantity  of  albumen,  will  attract  the  student's 
attention.  The  proportion  of  fibrin  present  varies  in  differ- 
ent parts  of  the  circulation,  and  it  is  not  easy  to  determine 
the  measure  of  its  variation ;  but  there  is  one  circumstance 
which  makes  it  seem  probable  that  the  fibrin  is  not  used  for 
the  manufacture  of  tissue,  but  is  a  product  resulting  from 
the  changes  effected  in  the  blood  by  circulating  among  the 
tissues ;  and  that  is,  that  the  blood  emerging  from  the  liver, 
after  being  subjected  to  the  action  of  that  organ,  is  no  longer 
spontaneously  coagulable,  and  only  yields  a  small  amount  of 
fibrin  after  violent  whipping  with  rods  (Beclard). 

The  fibrin  remains  fluid  while  the  blood  circulates  in  tho 
body,  yet  it  coagulates  almost  immediately  when  withdrawn 
from,  its  vessels,  and  still  more  speedily  when  stirred  than 
when  kept  at  rest,  unless  it  be  kept  fluid  by  reducing  tho 
temperature  to  the  freezing  point,  or  by  addition  of  certain 
foreign  matters.  This  fluidity  of  the  blood  within  the  vessels, 
and  coagulation  when  removed  from  them,  has  long  been  a 
puzzle  to  physiologists,  and  is  not  even  yet  fully  explained. 
But  there  is  one  point  which  is  certain,  namely,  that  coagula- 
tion is  the  result  of  the  mixing  together  of  two  different  sub- 
stances, both  of  them  albuminoids,  and  only  one  of  them 
present  in  the  liquor  sanguinis,  while  the  other,  which  is 
required  in  comparatively  very  small  quantity,  is  contained 
in  the  red  corpuscles.  The  fibrinous  element  of  the  liquor 
sanguinis  is,  on  this  account,  sometimes  termed  fibrinogen, 
while  the  element  furnished  by  the  corpuscles,  known  as 
paraglobulin,  gets  also  the  title  of  fibrinoplastin,  or  is  said 
to  exercise  a  fibrinoplastic  action.  The  necessity  for  the 
mixing  of  two  elements  before  coagulation  can  take  placo 
may  be  illustrated  by  tying  a  large  vein  of  an  ox  at  two 
places,  and  removing  the  included  portion  filled  with  blood. 
If  this  portion  of  vein  be  hung  up,  the  contained  blood  will 
remain  fluid,  but  the  corpuscles  will  fall  to  the  bottom.  If, 
after  that,  the  vein  be  opened,  so  as  to  allow  the  pure  liquor 
sanguinis  to  run  out,  it  will  be  found  that  the  liquid  so 
obtained  will  continue  fluid  for  any  length  of  time  in  any 
vessel,  and  however  much  it  may  be  stirred;  but  when  a  few 
Ved  blood  corpuscles  are  mixed  with  it,  it  coagulates  at  once. 


THE   BLOOD.  Ill 

Tliis  experiment  also  illustrates  another  point,  namely, 
that  while  contact  with  foreign  bodies  causes  the  red  cor- 
puscles to  part  with  their  paraglobulin,  the  wall  of  a  blood- 
vessel has  110  such  effect.  The  blood  will  remain  fluid  for 
days  in  the  veins  of  a  sheep's  trotter  got  from  the  butcher, 
and  yet  will  coagulate  immediately  when  the  veins  are  ripped 
open  with  scissors  (Lister).  We  do  not  know  the  explanation 
of  this,  and  we  do  not  know  to  what  the  formation  of  fibriiiogeii 
from  albumen  in  the  liquor  sanguinis  is  due ;  but  what  has 
been  said  is  sufficient  to  show  that  coagulation  is  not  a  vital 
process,  as  was  once  supposed,  but  is  a  change  of  a  chemical 
description. 

80.  The  red  corpuscles  consist  of  a  firm  stroma  with  a  sub- 
stance in  solution,  which  is  partly  composed  of  the  para- 
globulin  already  mentioned,  but  principally  of  a  coloured 
substance,    hcemoglobin,   which  is   an  albuminoid  with  the 
property  of  being  crystallizable,  the  form  of  crystal  varying 
in  different  animals.     The  colouring  matter  can  be  entirely 
separated  from  the  albuminoid,   but  not  without  chemical 
change,  the  product  obtained  being  termed  insoluble  hcematin, 
a  substance  remarkably  distinguished  by  yielding  more  than 
1 2  per  cent,  pure  oxide  of  iron  when  burned.    Iron  is  known 
in  medicine  as  a  most  powerful  tonic  in  debility  caused  by 
impoverishment  or  loss  of  blood,  and  this  is  in  some  measure 
explained  by  the  consideration  that  for  the  production  of 
blood,  it  is  an  essential  ingredient. 

Haemoglobin  is  principally  remarkable  as  the  substance 
which  gives  to  the  blood  its  power  of  absorbing  oxygen. 

81.  Blood  contains  in  its  composition  an  amount  of  gas, 
which,  when  liberated,  is  nearly  equal  to  half  the  volume  of 
liquid  from  which  it  has  been  set  free.     This  gas  can  be  ex- 
tracted by  means  of  the  air  pump,  part  of  it  easily,  and  the 
rest  with  the  aid  of  warmth.     It  contains  a  small  quantity  of 
nitrogen,  probably  introduced  in  the   lungs  from    the  ex- 
ternal aii-,  in  accordance  with  ordinary  physical  laws,  and 
not  of  any  physiological  importance.     But  the  great  bulk 
of  the  gas  consists  of  carbonic  acid  and  oxygen,  which  vary 
in  their  proportion  in  different  parts  of  the  circulation ;  the 
carbonic  acid  being,  however,  always  in  much  larger  volurue 
than  the  oxygen. 


112  ANIMAL   PHYSIOLOGY. 

It  has  already  been  pointed  out  that  throughout  the  body 
chemical  changes  are  constantly  taking  place,  in  which  oxygen 
combines  with  organic  matters,  and  that  carbonic  acid  is 
among  the  products.  This  oxygen  is  introduced  in  respiration, 
and  is  carried  by  the  blood  in  the  arteries  to  the  textures ; 
while  the  blood  which  returns  thence  by  the  veins  carries 
with  it,  back  to  the  lungs,  the  carbonic  acid  resulting  from 
the  processes  of  oxidation  which  have  taken  place  through- 
out the  body.  The  blood  going  to  the  textures,  or  what  is 
ordinarily  known  as  arterial  blood,  has  therefore  more  oxygen 
in  it  than  that  which  returns  by  the  veins,  and  the  venous 
blood  has  more  carbonic  acid  than  the  arterial.  There  is, 
however,  a  considerable  amount  of  oxygen  left  in  venous 
blood,  except  when  the  animal  is  killed  by  asphyxia,  that 
is  to  say,  stoppage  of  respiration  ;  and  the  amount  of  car- 
bonic acid  given  off  by  the  lungs,  is  only  a  small  proportion 
of  the  total  amount  contained  in  the  blood. 

82.  The  difference  in  the  gaseous  contents  of  the  blood  going 
to  the  textures,  and  that  which  returns  from  them,  is  accom- 
panied with  a  great  difference  of  colour.  When  blood  is 
allowed  to  flow  from  a  vein,  it  comes  in  a  stream  as  dark  as 
claret,  while  the  blood  which  comes  from  a  superficial  cut  is 
much  lighter,  and  what  spouts  from  a  wounded  artery  is  of 
a  bright  scarlet.  The  dark  blood  from  a  vein,  when  spilt  on 
the  ground,  becomes  bright  in  a  few  minutes,  exposure  to 
the  oxygen  of  the  air  sufficing  to  enable  it  to  part  with  car- 
bonic acid,  and  take  up  oxygen ;  and  scarlet  blood  exposed 
to  carbonic  acid  becomes  dark.  If  a  test  tube  be  filled  to 
about  a  fourth  from  the  top  with  defibrinated  blood,  such  as 
can  be  obtained  by  breaking  down  clot,  and  be  shaken  up  a 
few  times  so  as  to  enable  the  air  to  mix  with  it,  it  will 
become  bright  scarlet,  and  when  allowed  to  stand  for  some 
time  it  will  get  dark  again ;  when  shaken  a  second  time,  it 
will  again  grow  bright;  and  this  experiment  maybe  repeated 
on  the  same  specimen  of  blood  day  after  day.  The  same 
changes  may  be  exhibited  with  a  solution  of  the  colouring 
matter  of  the  red  corpuscles;  for  the  corpuscles  are  destroyed 
by  addition  of  water,  and  their  fluid  contents  are  set  loose, 
and  this  solution  alters  its  colour  on  exposure  to  oxygen  and 
carbonic  acid  alternately. 


THE    BLOOD.  113 

It  appears,  therefore,  in  the  first  place,  that  the*  difference 
in  colour  of  dark  and  scarlet  blood  depends,  at  least  partly, 
on  a  chemical  change  in  the  coloured  contents  of  the  cor- 
puscles; and  this  agrees  with  the  results  of  spectral  analysis, 
by  which  it  is  proved  that  the  colouring  matter,  or  cruorin 
as  it  is  sometimes  called,  if  arterial  blood,  is  a  different 
chemical  substance  from  that  of  venous  blood  (Stokes). 
In  the  second  place,  it  appears  that  the  hemoglobin  is 
the  oxygen-carrier  in  the  blood.  Indeed,  it  is  proved  by 
direct  experiment  that  serum  has  little  more  power  of  ab- 
sorbing oxygen  than  water  has.  "With  regard  to  the  carbonic 
acid  of  the  blood,  although  no  doubt  a  large  portion  of  it  is 
known  to  be  contained  in  the  serum,  it  seems  probable,  from 
the  effect  of  that  gas  on  the  colouring  matter,  that  the  por- 
tion which  is  removed  in  respiration  belongs  to  the  cor- 
puscles. 

83.  Before  leaving  this  subject,  it  may  be  well  to  notice  an 
exception  to  the  general  rule,  that  blood  returning  from  the 
textures  is  dark — not  only  is  that  sent  to  the  heart  from  the 
lungs  scarlet,  but  the  blood  returning  from  certain  glands  in 
action  is  of  the  same  tint.  Thus,  in  experiments  on  dogs  it 
has  been  found,  that  while  the  blood  in  the  veins  coming 
from  the  sub-maxillary  gland  is  dark  when  the  gland  is  at 
rest,  if  the  nerve  (chorda  tympani)  which  supplies  the  secreting 
structure  be  excited,  and  the  gland  thus  irritated  to  secrete 
saliva,  a  much  larger  quantity  of  blood  passes  through  the 
gland,  and  it  escapes  from  it  scarlet.  The  blood  returning  from 
the  kidneys  is  also  scarlet  as  long  as  urine  is  secreted,  but 
is  dark  when,  from  any  cause,  the  secretion  ceases.  In  both 
these  instances,  it  will  be  observed,  that  an  enormously  larger 
amount  of  blood  passes  through  the  organ  than  is  required 
for  the  nourishment  of  its  textures.  In  the  same  way,  if 
the  blood-vessels  of  a  rabbit's  head  are  paralysed  by  dividing 
the  sympathetic  nerve  in  the  neck,  in  consequence  of  the 
great  increase  of  blood  allowed  into  the  part,  a  portion  re- 
turns unaltered,  and  the  blood  is  found  red  in  the  jugular 
vein. 

H  H 


CHAPTER   IX. 
CIRCULATION. 

84.  WE  Lave  already  had  occasion  to  mention  that  the 
blood  circulates  through  the  body  in  a  system  of  close  vessels. 
It  is  propelled  by  the  heart  through  the  arteries  to  a  fine 
capillary  network,  whence  it  returns  to  the  heart  again  by 
the  veins. 

The  blood  which  has  circulated  in  the  tissues  requires  to  be 
aerated  to  reconvert  it  from  the  dark  to  the  scarlet  condition, 
before  it  can  be  allowed  to  go  to  the  tissues  again;  and  this 
is  managed  in  different  ways  in  different  animals.  In  fishes, 
the  blood  returning  from  the  system  is  propelled  by  the  heart 
into  the  gills,  and  from  them  right  on  into  the  system  again; 
passing  through  two  sets  of  capillaries,  one  in  the  gills 
and  the  other  in  the  system,  before  it  returns  to  the  heart. 
In  amphibians,  for  example  in  the  frog,  and  in  reptiles,  with 
the  exception  of  the  crocodiles,  the  blood  is  propelled  from 
the  heart  partly  into  the  respiratory  organs,  and  partly  into 
the  system,  and  returns  from  both  these  destinations  to  be 
mixed  in  the  heart;  and  this  mixture  of  scarlet  and  dark 
blood  is  what  circulates  again  both  in  the  system  and  respira- 
tory organs.  In  crocodiles,  none  but  dark  blood  is  sent  to 
the  lungs;  but  there  is  a  communication  by  which  part  of 
the  dark  blood  may  be  carried  back  into  the  system  along 
with  the  scarlet  stream.  In  warm-blooded  animals,  namely, 
birds  and  mammals,  the  whole  of  the  blood  returning  from 
the  system  is  sent  from  the  heart  to  the  lungs,  and  the  whole 
of  the  blood  returned  from  the  lungs  is  sent  into  the  system. 

In  fishes,  the  heart  consists  of  one  receiving  chamber  or 
auricle,  and  one  propulsive  chamber  or  ventricle.  In  the 
frog  and  the  turtle,  it  has  two  auricles,  one  receiving  dark 
blood  from  the  body,  and  the  other  red  blood  from  the  lungs, 


CIRCULATION. 


115 


and  these  discharge  their  contents  into  one  common  ventricle, 
which  propels  the  mixture  partly  into  the  lungs,  and  partly 
through  the  body.  la  warm-blooded  animals,  the  heart  is 
a  completely  double  organ,  consisting  of  two  auricles  and 
two  ventricles:  the  right  auricle  receives  the  dark  blood 
brought  back  from  the  tissues,  and  sends  it  into  the  right 
ventricle,  which  propels  it  through  the  lungs;  the  left  auricle 


Fig.  63. — HEART  AND  GREAT  VESSELS 
OF  FROG,  a,  Aorta ;  6,  venous  trunk 
carrying  dark  blood  to  c,  the  right 
auricle  of  the  heart;  d,  left  auricle 
receiving  aerated  blood  from  the 
lungs;  e,  ventricle  receiving  blood 
from  both  auricles,  and  propelling 
the  mixed  fluid  up  the  'truncus  ar- 
teriosus,  both  into  the  lungs  and 
the  system;  /,  left  lung. 


Fig.  62. — HEART  AND  GREAT 
VESSELS  OF  FISH,  a,  a,  a, 
Veins;  b,  b,  right  and  left 
extremities  of  the  single 
auricle  of  the  heart;  c,  ven- 
tricle of  heart;  d,  bulbus 
arteriosus  ;  e,  e,  branchial 
arteries  which  convey  the 
dark  blood  from  the  heart 
into  the  gills,  to  be  purified 
before  passing  on  into  the 
branchial  veins,  and  thence 
into  /,  the  aorta. 

receives  the  red  blood  returning  from  the  lungs,  and  passes 
the  pure  stream  on  into  the  left  ventricle  to  be  propelled  into 
the  tissues  of  the  body.  In  fishes,  as  well  as  in  warm-blooded 
animals,  only  red  blood  circulates  through  the  body;  but  in 


116  ANIMAL   PHYSIOLOGY. 

amphibians  and  reptiles,  while  the  heart  is  more  complex, 
the  circulation  is  less  perfect,  there  being  a  double  waste  of 
power  in  sending  part  of  the  dark  blood  into  the  body,  and 
part  of  the  red  blood  back  to  the  lungs,  which,  though  in 
a  manner  accounted  for  as  being  a  stage  of  progression 
toward  the  more  perfect  organ  found  in  higher  animals, 
might  have  been  difficult  to  explain,  if  it  could  have  been 
noted  by  an  observer  before  birds  and  mammals  appeared  on 
the  earth. 


Fig.  64. — DIAGRAM  OF  HITMAN  HEART  AND  VESSELS.  To  the  sides 
are  the  lungs  represented  in  outline;  and  above  and  below  are 
the  cut  ends  of  the  systemic  vessels.  The  arrows  indicate  the 
course  of  the  blood.  In  the  vessels  left  pale,  pure  blood  circu- 
lates; and  in  the  darkened  vessels,  impure  blood. 

85.  The  Heart  in  mammals,  as  will  be  seen  from  what  has 
been  said,  is  divisible  into  a  right  and  a  left  part,  each  of  which 
is  comparable  with  a  fish  heart,  consisting,  as  it  does,  of  an 
auricle  and  a  ventricle;  these  parts  are  completely  separate, 
one  from  the  other,  from  the  time  of  birth,  so  far  as  the 
blood  contained  in  them  is  concerned;  but  they  act  syn- 


THE   HEART.  117 

clironously,  and  are  structurally  one  heart.  Anatomically 
considered,  the  natural  division  of  the  heart  is  into  an 
auricular  and  ventricular  part,  separated  by  a  deep  sulcus, 
the  auriculo-ventricular  groove.  The  ventricular  part  is  a 
strong  musculaj1  structure  invested  completely  with  the  serous 
covering  of  the  pericardium,  and  unconnected  with  other 
viscera.  It  is  directed  downwards,  forwards,  and  to  the  left 
side,  resting  on  the  diaphragm^  in  man,  and  narrowing  to  the 
apex,  which  is  felt  beating  opposite  the  interval  between  the 
sixth  and  seventh  costal  cartilages  of  the  left  side.  The  apex 
is  formed  entirely  by  the  walls  of  the  left  ventricle,  which 
are  three  times  as  thick  as  those  of  the  right  ventricle,  the 
blood  requiring  much  greater  force  to  propel  it  through  the 
system  than  to  send  it  through  the  lungs;  and  if  the  ven- 
tricles be  cut  across,  the  section  of  the  left  ventricle  will  be 
seeii  to  be  circular,  while  the  right  is  curved  crescentically 
round  it. 

Above  the  auriculo-ventricular  groove,  ascending  from  the 
base  of  the  ventricles,  the  two  arterial  trunks  issuing  from 
those  two  cavities  lie  close  together  behind  the  breast-bone, 
each  twisted  somewhat  round  the  other.  That  which  rises 
from  the  right  ventricle  is  the  pulmonary  artery,  and  divides 
into  a  right  and  left  branch,  one  going  to  each  lung;  while 
the  systemic  artery,  arising  from  the  left  ventricle,  and  con- 
cealed at  its  origin  by  the  pulmonary  artery,  is  called  the 
aorta. 

The  auricles  have  exceedingly  thin  muscular  walls,  their 
whole  function  being  to  receive  the  blood,  which  continues 
pouring  in  during  the  contraction  of  the  ventricles,  and  to 
pass  it  into  them  through  the  large  auriculo-ventricular  aper- 
tures as  soon  as  they  relax.  They  lie  behind  and  to  the  sides 
of  the  arterial  trunks,  and  each  is  prolonged  into  a  pointed 
cul-de-sac  in  front,  which,  from  a  fancied  resemblance  to  a 
dog's  ear,  is  called  the  auricular  appendage;  and  these  ap- 
pendages have  given  their  name  of  auricles  to  the  cavities 
to  which  they  belong.  The  cavities  are  separated  one  from 
the  other  by  a  thin  septum,  which,  as  seen  from  the  right 
auricle,  presents  a  depression  and,  in  front  of  it,  a  crescentic 
border,  the  fossa  and  annulus  ovalis,  marking  the  position  of 
an  opening  which  exists,  and  is  made  use  of,  in  foetal  life, 


118  ANIMAL  PHYSIOLOGY. 

but  is  shut  up  after  birth.  In  the  rare  instances  in  which  it 
continues  after  birth  to  allow  blood  to  pass  through  it,  the 
circulation  of  dark  blood  in  the  system  is  the  result,  con- 
stituting the  disease  called  cyanosis,  and  destroying  life. 


Fig.  65. — RIGHT  SIDE  OF  THE  HEART,  a,  b,  Superior  and  inferior 
vena  cava  entering  the  right  auricle;  c,  Eustachian  valve;  d, 
annulus  ovalis;  e,  /,  anterior  and  posterior  cusps  of  the  tricuspid 
valve  descending  from  the  margin  of  the  auriculo-ventricular 
opening;  g,  pulmonary  artery  with  its  orifice  shut  by  distension 
of  the  three  pouches  of  its  semilunar  valve;  h,  aorta. 

The  right  auricle  receives  blood  in  two  streams  nearly  verti- 
cally opposite  one  another,  one  from  the  vena  cava  superior, 
bringing  the  blood  from  the  head  and  upper  limbs,  the  other 
from  the  vena  cava  inferior,  bringing  the  blood  from  the 
lower  limbs  and  greater  part  of  the  trunk;  while,  in  addition, 
the  blood  from  the  walls  of  the  heart  enters  by  oiie  consider- 
able and  several  smaller  orifices.  The  left  auricle  receives 
its  blood  by  streams  transversely  opposite  one  another, 
entering  by  the  pulmonary  veins  from  the  right  and  left 
lungs.  i  , 

86.  The  heart  can  be  seen  in  action  by  laying  open  a  frog, 
but  still  more  satisfactorily  in  the  chest  of  a  mammal.     The 


THE    HEART.  119 

auricles  are  seen  to  contract  first,  and  to  distend  the  ventri- 
cles with  blood;  the  ventricles  contract  immediately  after- 
wards, and  then  there  is  a  pause  before  the  auricles  become 
quite  distended  and  contract  again.  The  contraction  of  the. 
auricles  is  completed  in  about  a  third  of  the  time  taken  by 
the  ventricles  to  contract;  but  it  is  not  thorough,  for  it 
proceeds  in  a  wave  forwards  from  the  venous  trunks  to  the 
tips  of  the  appendages,  so  that  the  appendages  are  at  first 
distended,  and  when  in  turn  they  contract,  the  rest  of  the 
auricular  walls  are  already  relaxed.  On  account  of  this 
mode  of  contraction  of  the  auricles,  there  is  in  health  little 
tendency  of  the  blood  to  regurgitate  into  the  venous 
trunks ;  and  the  mouths  of  these  vessels  are  unguarded 
in  mammals,  although  protected  by  competent  valves  in 
other  'animals.  There  is  in  man  a  fold  of  membrane,  the 
Eustachian  valve,  in  front  of  the  vena  cava  inferior;  but  it 
can  have  little  action  as  a  valve  after  birth,  for  it  is  frequently 
nearly  absent  in  the  adult.  The  ventricles  contract  in  a 
different  way  from  the  auricles.  The  muscular  fibres  in  the 
middle  depth  of  their  walls  embrace  them  circularly,  while 
the  successively  deeper  and  more  superficial  layers  have  suc- 
cessively steeper  degrees  of  obliquity,  and  are  continuous 
one  with  another  both  at  base  and  apex ;  and  in  consequence 
of  this  arrangement,  these  cavities  are  contracted  throughout 
their  whole  extent  by  both  shortening  and  narrowing  at  the 
same  time,  till  they  are  completely  emptied.  But  no  matter 
how  completely  or  forcibly  the  ventricles  might  contract, 
they  would  make  but  an  inefficient  engine  of  propulsion  were 
there  not  some  means  of  preventing  the  blood  being  pushed, 
during  their  contraction,  back  into  the  auricles,  and  re- 
coiling, after  their  contraction,  back  into  them  from  the 
arteries.  Such  waste  of  power  is  prevented  by  the  presence 
of  valves,  which  guard  the  arterial  and  auriculo- ventricular 
orifices. 

87.  The  arterial  valves  guarding  the  entrances  into  the  pul- 
monary artery  and  aorta  are  named  semilunar,  because  they 
consist  each  of  three  delicate  pouches,  with  semilunar  attach- 
ments to  the  wall  of  the  artery.  The  pouches  are  placed  in  a 
circle,  with  their  mouths  turned  away  from  the  heart,  and 
are  pushed  flat  against  the  arterial  walls  when  the  blood  is 


120 


ANIMAL   PHYSIOLOGY. 


rushing  out  of  the  ventricles ;  but  as  soon  as  the  ventricular 
contraction  ceases,  and  the  elasticity  of  the  arteries  tends  to 

make  the  blood  recoil, 
they  are  filled  with  the 
blood  in  the  arteries,  the 
sides  of  each  are  pressed 
against  the  adjacent  sides 
of  the  two  others,  and  all 
three  reach  in  to  the  centre 
of  the  orifice,  so  as  effectu- 
ally to  block  it  up.  The 
action  of  these  valves  can 
be  studied  in  a  sheep's 
heart,  by  pouring  water 
into  the  cut  arteries,  when 
it  will  be  seen  that  not  a 
drop  passes  back  into  the 
ventricles. 

The  auriculo-ventricular 
valves   of   the    right    and 
left  sides  of  the  heart  are 
named     respectively     the 
tricuspid  and  the  bicuspid 
Fig.  60. — LEFT  SIDE  OF  THE  HEART,  or  mitral,  the  one  consist- 
The  pulmonary  artery  has  been  re-  ing  of  three  pointed  mem. 
moved.    An  arrow  is  passed  through  ,    *- 

the  aortic  orifice  between  the  semi-  branous  flaps  or  cusps,  and 
lunar  pouches  of  its  valve;  and  the  the  other  of  two.  The 
lower  end  of  the  arrow  rests  on  the  cusps  are  attached  at  the 
anterior  cusp  of  the  mitral  valve.  base  to  tlie  margins  Of  the 
a,  b.  Anterior  and  posterior  muscuh  ,  .  •  i 

papillares,  with  chorda*  tendmese  aunculo- ventricular  ^  on- 
passing  up  from  each  to  both  cusps;  fices,  and  are  kept  in  the 
c,  auricular  cavity.  interior  of  the  ventricles 

by  a  number  of  threads,  chordce  tendinece,  attached  to 
their  edges  and  backs,  and  fastened  at  the  other  end  to 
muscular  prominences,  the  musculi  papillares.  The  arrange- 
ments are  on  the  same  principle  in  both  valves,  but  may  be 
studied  best  on  the  mitral,  which  is  the  more  perfect  of  the 
two.  The  chords  tendineaD  of  each  cusp  are  divided  into 
two  sets ;  those  from  each  half  joining  with  those  of  the 
adjacent  half  of  the  other  cusp  to  be  inserted  into  one  mus- 


THE    HEART.  121 

culus  papillaris.  Thus  the  contraction  of  the  mttsculi  papillares 
not  only  prevents  the  cusps  from  being  flung  into  the  auricle, 
but  keeps  their  edges  in  apposition.  These  muscles  contract 
at  the  same  time  as  the  ventricular  wall  with  which  their 
fibres  are  continuous,  and  the  contraction  of  the  ventricle 
pushes  the  blood  against  the  backs  of  the  valves,  so  as  to 
bring  them  quite  together,  block  up  completely  the  passage 
into  the  auricle,  and  leave  the  blood  no  other  aperture  of 
exit  save  into  the  artery. 

88.  If  we  apply  our  ear  to  any  one's  chest,  we  find  that  the 
heart  makes  some  noise  in  its  action,  that  there  is  a  perpetual 
"  pit-pat,  pit-pat,"  or  recurrence  of  two  successive  sounds ; 
first  a  slightly  prolonged  sound,  then,  a  moment  afterwards, 
another  short  and  clear,  and  after  that  a  longer  interval 
before  the  first  sound  is  repeated.     The  first  sound  will  be 
found  to  occur  at  the  same  moment  as  the  beat  against  the 
chest,  and  nearly  at  the  same  moment  as  the  pulse  at  the 
wrist;  it  is  synchronous  also  with  the   systole  or  contrac- 
tion of  the  ventricles,  and  is  caused  by  the  vibration  of  the 
auriculo-ventricular  valves  when    suddenly   closed   by   the 
pressure  of  the  blood  on  them.     It  continues  audible  in  an 
animal  when  the  chest  has  been  laid  open,  so  does  the  second. 
The  second  sound  is  caused  by  the  closure  of  the  arterial 
valves,  as  has  been  experimentally  proved  by  introducing  a 
pair  of  needles,  one  into  the  pulmonary  artery,  and  the  other 
into  the  aorta,  so  as  to  prevent  the  valves  shutting,  and 
observing  that  at  once  the  sound  ceases. 

The  beating  of  the  heart  against  the  chest  is  called  the 
impulse,  and  is  caused  by  the  apex,  which  is  at  all  times  in 
contact  with  the  wall  of  the  chest,  being  pressed  against  it 
in  the  ventricular  contraction  by  jerking  upwards,  forwards, 
and  to  the  left.  This  movement  is  probably  caused  by  the 
arch  of  the  aorta  being  thrown  into  a  rigid  and  more  ex- 
panded curve  when  filled  with  blood,  while  it  is  fixed  in  its 
position  behind  ;  but  it  has  been  suggested  that  it  results 
from  the  disposition  of  the  muscular  fibres  of  the  ventricles. 

89.  The  frequency  of  the  heart's  pulsations  varies  in  health 
with  different  circumstances,  but  principally  according  to  age. 
In  infancy  the  beats  are  120  or  more  per  minute;   in  early 
life  they  quickly  dimmish  in  frequency ;  in  the  adult  they 


122  ANIMAL   PHYSIOLOGY 

are  about  75  in  the  male,  and  85  in  the  female ;  and  in  old 
age  the  average  is  lower. 

The  heart's  action  is  easily  influenced  in  its  regularity, 
strength,  and  rapidity  by  the  amount  of  blood  in  the  body, 
and  by  the  nervous  impressions  conveyed  from  other  parts. 
Thus,  it  may  be  weak  from  want  of  blood,  or,  when  the 
deficiency  is  sudden  or  great,  it  may  be  fluttering  or  irregular ; 
and,  on  the  other  hand,  the  rarer  phenomenon  is  occasionally 
observed  of  interference  with  the  heart's  action  dependent 
011  a  superabundance  of  blood.  Emotions  also,  and  conditions 
of  the  viscera,  send  impressions  through  the  nerves  which 
readily  disturb  the  heart.  But  it  is  important  to  observe 
that  the  rhythmic  action  may  continue  when  all  connection 
with  other  parts  has  been  cut  off.  A  turtle's  or  a  frog's  heart 
will  continue  to  beat  when  removed  from  the  body,  and  the 
successive  contractions  of  its  parts  will  continue  to  take 
place  in  regular  sequence,  even  though  there  is  no  longer 
any  blood  to  stimulate  it.  When  it  is  divided  vertically  the 
portions  continue  to  beat,  and  when,  divided  transversely 
the  rhythm  continues  in  the  basal  part,  but  is  lost  in  the 
apex.  There  are  not  only  numerous  nerves,  but  likewise 
minute  nerve-centres,  the  ganglia  of  fiemalc,  scattered  over 
the  heart ;  and  by  these,  kept  in  communication  with  one 
another  by  the  copious  nerves  in  the  auriculo-ventricuJar 
groove,  the  action  is  immediately  governed  (p.  216).  No  doubt 
it  is  difficult  to  understand  how  the  nerves  are  stimulated  to 
produce  rhythmic  contraction;  but  it  must  not  be  forgotten 
that  the  pulsation  of  the  heart  is  only  one  of  a  great  number 
of  instances  of  periodicity  in  nervous  action,  and  that  any 
nervous  action  frequently  repeated  has  a  tendency  to  go  oil 
recurring. 

90.  The  Arteries,  into  which  the  blood  is  sent  by  the 
heart,  are  a  series  of  branching,  elastic,  and  contractile 
tubes.  They  have  a  smooth  internal  lining,  and  externally 
have  a  tough  felted  coat  of  areolar  tissue ;'  but  the  main 
thickness  of  their  walls  consists  of  a  middle  coat  of  elastic 
and  muscular  fibres  intermixed  and  arranged  circularly,  lying 
among  meshes  of  elastic  membrane.  In  the  larger  arteries 
the  muscular  fibres  are  exceedingly  small,  and  the  elastic 
fibres  abundant;  but  as  the  vessels  get  smaller  there  is  a 


THE   ARTERIES. 


123 


greater  development  of  muscular   tissue,    and   less  of  the 
elastic,  until  in  the  minute  arterioles  the  elastic  tissue  dis- 


Fig.  67.— -ARTEBIAL  SYSTEM, 


124  ANIMAL    PHYSIOLOGY. 

appears  altogether,  and  the  middle  coat  consists  of  inusclG 
only. 

The  advantage  of  the  elasticity  of  the  arterial  walls  may 
be  easily  illustrated.  If  a  glass  tube  have  a  nozzle  fastened 
into  it  at  one  end,  and  at  the  other  be  fitted  to  the  stop-cock 
of  a  water  pipe,  and  if  the  water  be  turned  on  and  off  alter- 
nately so  as  to  imitate  the  repeated  discharges  of  blood 
from  the  ventricles,  the  water  will  emerge  from  the  nozzle 
in  jets,  which  will  cease  instantaneously  each  time  that  it  is 
turned  off.  But  if  the  same  experiment  be  made  with  a 
long  india-rubber  tube  instead  of  a  glass  one,  the  water  will 
spring  from  the  nozzle  in  a  continuous  flow,  notwithstanding 
the  interrupted  manner  in  which  it  is  admitted  to  the  tube  ; 
and  if  the  experiment  be  varied  so  that  the  glass  and  the 
india-rubber  tube  shall  both  be  filled  from  the  top  at  the 
game  time,  while  the  nozzles  on  the  two  tubes  are  of  the  same 
size,  the  elastic  one  will  discharge  in  a  given  time  a  much 
larger  quantity  of  water  than  the  one  which  is  rigid.  In  the 
rigid  tube  there  is  great  loss  of  force  by  friction ;  while  in 
the  elastic  tube,  as  each  fresh  jet  of  fluid  enters,  the  walls 
are  distended,  and  as  it  ceases  they  recover,  and  give  their 
contents  a  fresh  propulsion  onwards  in  a  second  wave,  which 
distends  the  tube  further  on ;  and  thus,  after  traversing  a 
sufficient  length  of  tube,  the  interrupted  stream  is  converted 
into  one  which  is  continuous.  This  is  precisely  what  happens 
in  the  arteries.  When  a  large  artery  is  divided,  the  blood 
comes  in  separate  abrupt  jets  with  well  marked  intervals 
between  ;  in  smaller  arteries  the  duration  of  the  jets  is  longer 
and  the  intervals  are  shorter,  and  from  little  twigs  the  blood 
spouts  out  in  an  almost  continuous  stream. 

While  the  elasticity  of  the  arteries  thus  converts  the  sepa- 
rate gushes  of  blood  from  the  heart  into  one  continuous  flow 
before  the  capillaries  are  reached,  their  contractility,  derived 
from  their  muscular  fibres,  determines  the  amount  of  blood 
which  is  sent  at  different  times  to  each  part.  H  Their  contrac- 
tion is  not  of  the  vermicular  description,  but  purely  tonic : 
they  do  not  assist  the  forward  movement  of  the  blood  by 
propelling  it  onwards,  but,  by  varying  in  diameter  at  different 
times,  they  allow  more  or  less  blood  to  pass  through  them. 
The  muscular  fibres  are  governed  by  nerves,  termed  vaso- 


THE    ARTERIES,  125 

motor:  these  are  found  in  the  sympathetic  trunks  (p.  215), 
and  their  action  may  be  illustrated  by  dividing  the  sympa- 
thetic nerve  of  a  rabbit  in  the  neck,  when  immediately 
the  ear  of  the  side  experimented  on  gets  red,  and  the  whole 
of  that  side  of  the  head  becomes  warmer  than  the  other; 
the  reason  being  that  the  paralysed  arterioles  no  longer 
resist  the  entrance  of  the  red  and  warm  blood,  but  allow  it 
to  distend  them.  This  condition  is  the  same  as  takes  place 
in  blushing ,  only  in  blushing  the  withdrawal  of  the  nervous 
stimulus  is  temporary,  caused  by  the  communication  of  a  f 
disturbing  influence  from  the  brain,  the  result  of  emotion. 

91.  The  pulse  in  the  arteries  is  caused  by  their  distension 
and  elongation  under  the  pressure  exerted  by  the  rush  of  blood 
with  each  beat  of  the  heart,  but  can  only  be  felt  in  those 
positions  in  which  an  artery  lies  near  some  firm  structure 
against  which  it  can  be  pressed.  With  the  aid  of  instru- 
ments it  can  be  shown  that  it  is  communicated  with  great 
rapidity  to  the  whole  arterial  system,  the  smallest  arteries 
pulsating  within  a  sixth  of  a  second  after  the  largest.  The 
blood  already  in  the  arteries  is  pushed  on  by  each  new  quan- 
tity thrown  in  from  the  hearty  the  velocity  with  which  the 
blood  travels  being  far  less  than  that  with  which  the  pulse 
is  communicated.  The  actual  rate  of  movement  of  the  blood 
can  be  observed  in  the  arteries  of  animals  by  the  insertion 
of  an  instrument  for  the  purpose  between  the  ends  of  a 
divided  vessel,  and  it  is  calculated  from  such  experiments, 
that  in  man  the  mean  velocity  is  about  ten  inches  per  second 
in  the  carotid,  and  about  two  and  a  quarter  in  the  foot,  the 
rate  of  flow  being  much  slower  in  small  arteries  than  in 
large.  The  reason  why  it  is  slower  is  to  be  found  in  a  pecu- 
liarity in  the  branching  of  arteries;  for  in  almost  all  instances 
in  which  an  artery  divides,  the  united  areas  of  the  divisions 
are  greater  than  the  area  of  the  parent  trunk;  and,  conse- 
quently, the  total  area  of  the  combined  arterial  channels  in- 
creases rapidly  the  farther  the  distance  from  the  heart. 

While  the  frequency  of  the  pulse  corresponds  with  that  of 
the  heart's  contractions,  its  character  depends  on  a  variety  of 
circumstances,  of  which  the  chief  are  the  amount  of  blood 
in  the  body,  the  vigour  and  regularity  of  the  heart's  action, 
the  degree  to  which  the  muscular  fibres  of  the  arterial 


126 


ANIMAL   PHYSIOLOGY. 


wall  are  contracted,  and  so  offer  resistance  to  the  heart. 
Some  of  the  peculiarities  of  the  pulse,  which  cannot  be 
appreciated  by  pressure  of  the  finger  on  the  wrist,  are 
exhibited  with  the  aid  of  the  sphygmograph  (Marey),  an 
instrument  which  is  fastened  to  the  wrist,  and  in  which  a 
spring,  pressed  against  the  radial  artery,  causes  a  light 
lever,  carrying  a  pen,  to  move  up  and  down.  The  pen  is 
in  contact  with  a  slip  of  paper  or  smoked  glass  set  in 
motion  by  clock-work,  and  produces  a  tracing  which 
indicates  the  pulsations  by  elevations,  and  the  element  of 
time  by  horizontal  distance.  Such  a  tracing  shows,  that  in 
health  the  distension  of  the  vessel  takes  place  with  almost 
instantaneous  suddenness,  commencing  and  finishing  abruptly ; 


1.1 


3. 


Fig.  G8. — SPHYGMOGRAPHIC  TRACINGS  of  the  pulses  of  three  persona 
all  healthy.  In  1,  the  arterial  resistance  is  greatest;  in  2,  the 
dilatation  of  the  vessel  has  taken  place  with  such  force  as  to  jerk 
the  lever  of  the  instrument  from  its  rest,  and  hence  the  sharp 
points  at  the  tops  of  the  waves;  3,  is  a  distinctly  dicrotous  pulse. 

while  the  gradual  character  of  the  recoil  is  shown  by  its 
making  a  long  sloping  line.  When  the  arterial  resistance  is 
great,  as  it  is  in  the  most  robust  health,  it  counteracts  the 
distending  impulse  given  by  the  heart,  so  that  the  rise  of  the 
tracing  is  not  so  considerable  as  it  would  otherwise  be;  and 
in  these  circumstances  there  is  a  moment's  continuance  of  the 
distension,  then  a  gradual  and  but  slightly  undulating  de- 
scent. But  when  the  arterial  resistance  is  slight,  the  secondary 
distending  impulses  given  by  the  elastic  recoil  of  the  larger 
vessels  produce  more  effect  on  the  tracing,  and  one  particular 
rise  becomes  prominent,  which  appears  to  be  caused  by  the 
walls  of  the  commencement  of  the  aorta,  redistended  by  the 


THE    CAPILLARIES. 


127 


blood  thrown  back  on  the  aortic  valves,   again   recoiling. 
Such  a  pulse  is  said  to  be  dicrotous. 

92.  The  Capillaries  are  the  smallest  blood-vessels,  those 
through  the  walls  of  which  materials  pass  to  and  from  the 
tissues,  and  so  delicate  that,  as  has  already  been  pointed  out, 
even  blood  corpuscles  are  able,  without  injury  to  the  walls,  to 
escape  from  them  into  the  parts  around.  They  vary  from 
•^(jo-  to  jj^Vo-  inck  in  diameter,  and  are  arranged  like  the 
meshes  of  a  net.  The  meshes  vary  in  size  and  form  in 
different  localities ;  for  the  most  part  they  are  polygonal ;  in 
the  papillae  of  the  skin  they  are  in  loops ;  in  muscle  they 
are  oblong;  and  in  the  lung  they  are  circular,  with  the 
diameters  of  the  circles  little  greater  than  the  breadth  of  the 
capillaries  between  which  they  lie.  In  some  tissues  they 
can  be  seen  under  the  microscope  without  previous  prepara- 
tion; and  they  exhibit  the  appearance  of  a  homogeneous 
membranous  wall  with  oval  nuclei  imbedded  in  it,  and 
projecting  to  the  outside.  With  the  aid  of  a  weak  solution 
of  nitrate  of  silver,  a  delicate  lining  of  epithelium,  or  endo- 
thelium  as  it  is  sometimes  called,  is  brought  into  view;  but 
it  must  not  be  supposed  that  the  nuclei  mentioned  belong  to 
that  lining. 


Fig.  69. — CAPILLARIES,  highly  magnified.     A,  exhibits  the  nuclei; 
B,  the  endothelmm  as  displayed  by  means  of  nitrate  of  silver. 

The  blood  can  be  seen  circulating  in  the  capillaries  of  the 
web  of  a  frog's  foot,  a  tadpole's  tail,  or  a  bat's  wing,  without 


128  ANIMAL    rilYSIOLOGY. 

injury  to  the  animal ;  and  may  be  still  better  studied  in 
some  internal  parts  of  small  animals  operated  on  for  the 
purpose.  Furthermore,  by  gazing  steadily  at  a  bright  field, 
and  moving  a  finger  rapidly  in  front  of  the  eye,  some  persons 
are  able  to  bring  into  view  the  blood  corpuscles  coursing  in 
the  capillaries  of  the  retina  in  their  own  eyes  (p.  251).  From 
such  data  as  these,  the  calculation  is  made  that  the  blood 
moves  in  the  capillaries  in  the  human  subject  at  the  rate  of 
one  or  two  inches  per  minute.  The  movement  looks  much 
more  rapid  when  seen  under  the  microscope  in  a  frog's  foot; 
but  the  reason  of  that  is,  that  the  distance  which  the  cor- 
puscles travel  being  magnified,  the  apparent  rate  of  motion 
is  proportionately  increased ;  because  the  rate  of  movement 
is  the  distance  travelled  in  a  given  time. 

93.  The  Veins  begin  by  radicles  from  the  capillaries,  in 
like  manner  as  the  arteries  end  in  these  vessels.  The  blood 
moves  in  them  from  the  capillaries  towards  the  heart,  and 
their  course  on  that  account  is  described  from  twig  to  trunk 
like  the  course  of  a  river.  They  are  larger  than  the  corre- 
sponding branches  of  arteries,  and  in  the  limbs  they  are  more 
numerous.  Thus,  in  the  lower  limb  below  the  knee,  and  in 
the  upper  limb  below  the  armpit,  the  main  arteries  are 
accompanied  with  vence  comites,  that  is  to  say,  two  or  more 
veins  frequently  communicating;  and  there  are,  besides, 
large  veins  beneath  the  skin  without  any  corresponding 
artery.  The  walls  of  veins  are  much  thinner  than  arterial 
walls,  owing  to  their  having  the  middle  or  muscular  and 
elastic  tunic  so  slightly  developed  that  their  principal  thick- 
ness consists  of  the  external  or  felted  coat;  and  thus  it 
happens  that  in  a  wound,  an  artery  cut  across  forthwith 
contracts  so  as  to  lessen  its  apparent  diameter,  while  a  vein 
gapes  and  continues  as  large  as  ever. 

The  veins  give  little  or  no  assistance  to  the  flow  of  blood 
by  elastic  recoil,  and,  indeed,  become  easily  distended  to  an 
undue  extent;  but  they  present  a  peculiar  provision  to  prevent 
the  accumulation  of  pressure  within  them,  and  regurgitation 
backwards,  for  they  are  provided  here  and  there  with  valves. 
These  valves  are  on  the  same  principle  as  the  arterial  valves  of 
the  heart,  consisting  of  semilunar  pouches;  but  the  pouches 
look  towards  the  heart,  and  instead  of  there  being  three  of 


THE   VEINS. 


129 


A 


them  together,  there  are  only  two.  They  are  very  delicate, 
consisting  of  folds  of  the  lining  membrane  of  the  vessel,  and 
are  quite  transparent;  but  when 
liquid  is  injected  from  the  direc- 
tion of  the  heart,  it  is  effectually 
prevented  by  them  from  passing 
back  to  the  twigs.  They  are 
nearly  absent  from  the  head  and 
neck,  and  are  most  abundant  in 
the  lower  limbs.  By  prevent- 
ing regurgitation,  they  convert 
the  accidental  pressure  of  sur- 
rounding parts  into  an  auxiliary 
of  the  circulation,  pushing  the 
blood  onwards,  but  incapable  of 
pushing  it  backwards.  Such 

pressure  may,  however,  easily  Fig.  70. — VENOUS  VALVES.  A, 
be  sufficiently  great  to  prevent 
the  entrance  of  blood  into  a 
vein;  thus  in  letting  blood  from 
the  arm  it  is  customary  to  make 
the  patient  exercise  the  fingers 
sufficiently  to  keep  up  pressure  by  contraction  of  the  flexor 
muscles  in  the  forearm  on  the  deep  veins,  and  so  compel  the 
return  of  the  blood  by  the  superficial  set. 

It  will  be  perceived  from  what  has  been  said,  that  the 
rate  of  flow  of  the  blood  in  individual  veins  is  very  variable. 
Looking,  however,  at  the  venous  system  as  a  whole,  it  will 
be  easily  understood  that  it  pours  into  the  heart,  in  a  given 
time,  exactly  the  same  amount  of  blood  as  is  discharged  into 
the  arteries;  and  as  the  sectional  area  of  the  veins  is  every- 
where greater  than  that  of  the  corresponding  arteries,  the 
flow  of  blood  within  them  is  in  the  same  proportion  slower, 
probably  nowhere  more  than  half  as  fast;  but  as,  in  the 
arteries,  the  velocity  of  the  blood  is  less  the  nearer  it 
approaches  the  capillaries,  on  account  of  the  larger  sectional 
area  of  the  total  number  of  vessels  in  which  it  is  distributed, 
so  it  again  increases  in  the  passage  from  the  sir  nil  to  the 
large  veins. 

94.  The  pressure  or  force  with  which  the  blood  is  urged  on  its 


a  vein  laid  open  to  show  the 
two  pouches  of  a  valve.  B, 
an  unopened  vein,  exhibits  at 
a,  the  dilatation  opposite  a 
valve ;  and  at  5,  a  closed 
valve  seen  from  below. 


130  ANIMAL   PHYSIOLOGY. 

course  by  the  heart  must  not  be  confused  with,  its  velocity. 
The  velocity  is  at  its  mminium  in  the  capillaries ;  the  pressure 
diminishes  from  quitting  the  heart  till  the  return  to  it,  being 
dissipated  by  the  friction  of  the  tissues  which  resist  it.  From 
experiments  on  the  lower  animals,  it  is  calculated  to  be  equal 
in  large  arteries,  such  as  the  carotid,  to  the  support  of  a 
column  of  mercury  more  than  six  inches  high,  and  in  small 
arteries,  like  those  of  the  foot,  to  be  about  a  fourth  less: 
while  in  the  veins,  after  having  experienced  the  resistance  of 
the  tissues  in  the  capillaries,  it  is  only  about  a  twelfth  of 
what  it  is  in  the  arteries.  These  observations,  together  with 
the  fact  that  defTbrinated  blood  has  been  injected  through  the 
body  of  a  dog  with  less  pressure  than  that  exerted  by  the 
heart  (Sharpey),  point  out  that  the  heart  is  the  motive 
power  which  causes  the  blood  to  flow  through  the  whole 
system.  It  must  not,  however,  on  that  account  be  supposed 
that  the  tissues  have  no  influence  whatever  on  the  circula- 
tion, for  we  have  proof  to  the  contrary  in  the  fact  that  in 
interference  with  respiration,  the  unaerated  blood  fails  to 
pass  the  capillaries,  and  that  in  inflammations,  examined 
microscopically  in  the  web  of  the  frog's  foot,  blood  corpuscles 
are  seen  arrested  in  their  course  without  any  obstruction 
existing  in  the  channel  beyond. 

95.  The  time  required  for  a  portion  of  the  blood  to  be  carried 
through  the  whole  circulation,  has  been  made  the  subject  of 
most  interesting  experiments.  An  easily  detected  substance, 
such  as  ferrocyanide  of  potassium,  is  introduced  into  a  vein 
on  one  side  of  the  neck  of  an  animal,  and  the  time  noted  which 
elapses  before  it  is  present  in  the  blood  allowed  to  flow 
from  the  corresponding  vein  on  the  other  side.  The  substance 
introduced  has  to  pass  through  the  heart  and  lungs,  and  some 
.  part  of  the  head  or  neck,  before  it  can  reach  the  aperture 
where  it  is  sought  for,  and  thus  makes  a  circuit  through  both 
pulmonary  and  systemic  circulation.  In  the  horse,  such  a 
circulation  is  completed  in  little  more  than  half  a  minute, 
and  in  smaller  animals  in  a  much  shorter  time.  Small 
animals  have  the  pulse  rapid;  and  the  rule  may  be  laid 
down  that  a  complete  circulation  takes  place  in  from  20  to 
30  beats  of  the  heart,  This  may  well  appear  incredible  at 
first,  when  it  is  considered  how  slowly  the  blood  moves  in 


PORTAL   SYSTEM. 


131 


the  capillaries,  but  the  explanation  is  found  by  taking  into 
account  the  exceedingly  short  distance  of  capillary  circula- 
tion traversed  by  each  portion  of  blood,  probably  in  no  case 
exceeding  the  tenth  of  an  inch. 


Fig.  71. — VENOUS  SYSTEM,  diagrammatic  view.  «,  Trachea  dividing 
into  the  two  bronchi;  6,  aorta  dividing  into  the  two  common 
iliac  arteries,  which  again  divide  into  the  external  and  internal 
iliacs ;  c,  c,  kidneys,  with  the  renal  veins  emerging  from  them ; 
d,  liver ;  e,  spleen  ;  /,  portion  of  intestine,  with  mesenteric  vein, 
proceeding  from  it  to  join  the  splenic  and  form  the  portal  vein, 
which  branches  in  the  liver ;  g,  inferior  vena  cava  receiving  the 
hepatic  veins  emerging  from  the  liver ;  h,  obliterated  umbilical 
vein;  i,  obliterated  ductus  venosus;  k,  superior  vena  cava  formed 
by  union  of  the  two  innominate  or  brachio-cephalic  veins ;  Z,  the 
right  vena  azygos  joined  by  the  left. 

96.  Portal  System. — Before  leaving  the  subject  of  the 
circulation,  it  remains  to  be  pointed  out  that  there  are 
exceptions  to  the  rule  that  the  arteries  continually  divide 


132  ANIMAL   PHYSIOLOGY. 

till  they  reach  the  capillaries,  and  that  the  veins  emerging 
from  these  bring  the  blood  directly  back  to  the  heart. 
Looking  at  vertebrate  animals  generally,  one  finds  many 
instances  of  arteries  breaking  up  into  small  branches  which 
reunite  before  reaching  the  capillaries,  and  such  an  arrange- 
ment is  called  a  rete  mirabile.  There  is  only  one  instance  of 
such  a  thing  occurring  in  the  human  subject,  namely,  in  the 
Malpighian  corpuscles  of  the  kidney.  But  there  is  a  notable 
instance,  occurring  in  man  and  all  vertebrate  animals,  of  a 
venous  trunk  branching  again  into  twigs,  which  open  into  a 
second  set  of  capillaries;  and  that  is  the  portal  vein.  The 
portal  vein  receives  all  the  blood  returning  from  the  stomach, 
intestines,  and  spleen,  and  divides  into  a  right  and  a  left 
branch,  which  enter^the  liver,  and  break  up  into  branches 
which  pour  their  contents  into  the  capillaries  of  that  organ, 
and  then  discharge  tneir  blood  into  the  hepatic  veins,  which 
open  into  the  vena  cava  inferior.  Thus  all  the  blood  which 
goes  to  the  stomach  and  intestines  has  to  pass  through  two 
sets  of  capillaries  before  returning  to  the  heart,  and  this  is 
the  blood  on  which  the  liver  exercises  its  purifying  power. 


CHAPTER   X. 
RESPIRATION  AND  TEMPERATURE. 

97.  THE  object  of  respiration  is  to  liberate  the  carbonic 
acid  accumulated  in  the  blood  returning  from  the  tissues,  and 
to  take  in  a  fresh  supply  of  oxygen,  which,  passing  to  the 
tissues,  is  used  in  chemical  decompositions,  by  which  more 
carbonic  acid  is  produced.  Respiration,  therefore,  is  not  a 
process  of  combustion,  but  it  affords  an  index  of  the  amount 
of  combustion  taking  place  in  the  tissues.  It  consists  essen- 
tially of  an  interchange  of  gases  between  the  blood  and  the 
medium  in  which  the  animal  lives,  and  requires  that  these 
two  should  be  brought  into  as  close  contact  as  possible. 

This  contact  is  achieved  in  some  animals  by  introducing 
the  medium  into  the  body,  and  conveying  it  to  the  tissues. 
Thus,  in  star  fishes  there  is  a  water-vascular  system,  and  in 
insects  there  are  air  tubes  or  trackece  kept  open  by  a  spiral 
thread  coiled  round  them,  which,  opening  by  stigmata  on  the 
sides,  ramify  throughout  the  whole  body,  and  are  emptied 
and  refilled  with  air  by  pulsatile  movements  of  the  abdomen. 
But  in  the  majority  of  animals,  including  all  vertebrates, 
the  blood  is  brought  into  contact  with  the  surrounding 
medium  in  a  special  organ  devoted  to  the  purpose;  and  this 
in  water-breathing  animals  consists  of  gills  or  projecting 
organs  with  blood-vessels  on  the  surface,  while  in  air-breath- 
ing animals  it  consists  of  lungs  or  bags  into  which  the  air 
is  introduced. 

In  frogs  and  serpents,  the  lungs  are  simple  pouches  with 
shallow  recesses  round  about,  the  partitions  of  which  project 
into  the  interior;  and  these  pouches  are  each  opened  abruptly 
into  by  a  main  air  tube  or  bronchus.  In  turtles  and  croco- 
diles the  air  tubes  are  branched,  and  each  branch  opens  into 
a  cavity  in  the  heart  of  a  sponge  of  ramifying  recesses  ; 


134 


ANIMAL   PHYSIOLOGY. 


while  in  birds  there  are  no  longer  any  dilated  cavities,  but 
every  air  tube  is  surrounded  with  a  system  of  air  cells,  com- 
pletely separated  by  septa  of  connective  tissue  from  those 
which  surround  others.  In  the  lungs  of  man  and  other 
mammals,  the  air  tubes  go  on  dividing  and  subdividing  till 
they  terminate  in  minute  tubules,  which  open  into  irregular 
passages,  surrounded  with  air  cells :  and  in  the  manner  of 
their  development  they  exhibit  both  the  modes  of  increase 
in  complexity  observed  in  the  zoological  series;  for  they  take 
origin  in  the  embryo  as  a  pair  of  simple  pouches  coming  off 
from  the  throat,  and  these  branch  out  into  smaller  pouches 
budding  in  like  manner  till  lobules  are  formed,  which, 
instead  of  branching  outwards,  have  septa  growing  inwards 
into  their  cavities. 


73. — DUCK'S  LUNG;   section,  magni- 
fied twenty  diameters. 


Pig.  72. — FROG'S  LUNG, 
opened  from  behind, 
enlarged. 

98.  The  main  air  tube,  the  trachea  or  windpipe,  is  about  four 
and  a  half  inches  long,  and  is  surmounted  by  the  larynx,  the 
part  in  which  the  glottis  is  placed,  and  which  constitutes  the 
organ  of  voice.  The  constant  patency  of  the  trachea  is 
insured  by  a  series  of  cartilaginous  hoops,  which  in  some 
animals  form  complete  rings,  but  in  the  human  subject  are 
deficient  behind.  This  tube  divides  below  into  a  right  and 
left  bronchus  of  similar  structure;  and  each  of  these  divides 


WINDPIPE  AND   UJNGS. 


135 


and  subdivides  into  smaller  and  smaller  bronchial  tubes,  the 
larger  of  which  have  crescentic  cartilages  arranged  in  their 
walls,  not,  as  in  the  trachea,  one  directly  over  another,  but 
in  such  a  way  as  to  maintain  the  cylindrical  form  of  the 
tube  011  every  aspect,  while  in  the  smaller  these  cartilages 
degenerate  to  irregular  nodules,  and  disappear. 


Fig.  74. — HUMAN  WINDPIPE  AND  LUNGS,  a,  Hyoid  bone;  &,  c, 
thyroid  and  cricoid  cartilages  of  larynx  ;  d,  trachea  dividing 
inferiorly  into  right  and  left  bronchus;  e,  root  of  left  lung,  the 
pulmonary  artery  and  vein  cut  across;  /,  /,  the  bases  of  the 
lungs,  which  rest  011  the  diaphragm;  g,  g,  portions  of  the  anterior 
margins,  which  in  the  body  reach  to  the  middle  line,  and  have 
only  folds  of  pleura  between  them.  The  right  lung  is  seen  to 
have  three  lobes,  the  left  two;  the  right  is  shorter  than  the  left, 
and  the  anterior  part  of  the  left  is  hollowed  out  opposite  the 
position  of  the  apex  of  the  heart. 

In  all  these  tubes,  except  the  smallest,  there  is  a  longi- 


136  ANIMAL  PHYSIOLOGY. 

tudinal  arrangement  of  elastic  fibrous  tissue,  enabling  them 
to  elongate  when  required  by  the  stretching  of  the  neck  or 
the  expansion  of  the  lungs;  also  transverse  muscular  fibres, 
confined,  in  the  trachea  and  bronchi,  to  the  back  part  where 
the  cartilages  are  deficient,  but  circular  in  the  other  tubes, 
and  capable  by  their  contraction  of  modifying  the  freedom 
of  entrance  of  air  into  the  lungs,  as  is  strikingly  exemplified 
in  asthma,  which  consists  of  spasms  of  these  muscles  to  such 
extent  as  to  produce  difficulty  of  breathing.  The  mucous 
membrane  is  furnished  with  mucous  glands,  and  lined  with 
ciliated  columnar  epithelium.  But  the  bronchial  tubes  of 
smallest  size,  reduced  to  -^  inch  in  diameter,  have  homo- 
geneous walls  with  a  simple  squamous  lining,  and  each  of 
these  terminates  in  an  ultimate  lobule  or  infundibulum,  con- 
sisting, as  has  been  already  said,  of  an  irregular  passage, 
surrounded  with  air  cells.  These  air  cells  or  locules  are  cup- 
shaped  depressions,  consisting  of  a  framework  of  fine  elastic 
tissue,  and  one  of  the  closest  capillary  networks  in  the  body, 
in  which  circulates  the  blood  sent  to  the  lungs  by  the  pul- 
monary artery;  and  in  order  to  secure  the  greatest  possible 
contact  of  the  blood  with  the  air,  the  septa  between  the 
locules  present  only  one  layer  of  capillaries  exposed  on  both 
sides,  and  protected  only  by  a  very  delicate  epithelium. 


Fig.  75. — CAST  OF  INFUNDIBULA     Fig.  76. — AIR  CELLS,  with  capil- 
aT5 ;  from  Henle.  lary  blood-vessels  injected,  \°. 

The  ultimate  lobules  are  united  in  groups  to  form  larger 
lobules,  from  a  quarter  to  half  an  inch  in  diameter,  very 
closely  united  one  to  another,  but  with  outlines  which,  in 
those  of  the  surface  of  the  lung,  can  be  easily  distinguished, 


MECHANISM    OF    RESPIRATION.  137 

particularly  in  the  human  subject,  where  they  are  usually 

more  or  less  marked  by  the  deposit  of  black  pigment.     The 

tissue  between  the  lobules  and  that  of  the  bronchial  tubes 

require,    like    all    other 

tissue,  oxygenated  blood 

for   their   nourishment ;  •• 

and   as   that  which  ar-  \_ 

rives  by  the  pulmonary  • 

artery  is  unfit  for  use,  ^ 

they  receive  their  supply  ^f^* 

from      small      systemic  [ 

branches  called  the  broil-  • 

chial  arteries. 

Each    lung    is    con 
pletely  invested  with  a ' 
serous    membrane,    the 
pleura,,  the  visceral  layer 
of  which  is  adherent  to 
its    surface,    while    the 
parietal  layer  is  reflected  Fig.   77. — LOBULES  OP  HUMAN    LUNG, 
from  what  is  called  the         partially  separated;  natural  size. 
root  of  the  lung,  where  the  bronchus  and  blood-vessels  enter, 
to  the  walls  of  the  chest,  the  outer  surface  of  the  peri- 
cardium, and  the  upper  surface  of  the  diaphragm. 

99.  The  means  by  which  the  air  is  introduced  into  the  lungs 
and  expelled  therefrom,  is  similar  to  that  by  which  it  is 
drawn  into  and  forced  out  from,  a  concertina.     The  interior 
of  the  lungs   communicates  freely  with  the  air  outside  by 
means  of  the  windpipe,  and  when  the  capacity  of  the  cavity 
containing  them  is  enlarged,  the  air  passes  in,  while,  when 
the  capacity  is  diminished,  a  quantity  of  air  is  expelled. 
This  is  the  principle  of  the  mechanism,  of  respiration  in  the 
majority  of  animals;  but  in  frogs  the  want  of  osseous  thoracic 
walls,  and  in  turtles  their  rigidity,  renders  it  impracticable; 
and  therefore  both  these  animals  pump  the  air  down  into 
their  lungs  by  a  motion  of  a  swallowing  description,  which 
may  be  seen  constantly  going  on  in  their  throats. 

100.  The  expansion  and  diminution  of  the  chest  in  respira- 
tion affects  the  whole  thoracic  cavity,  containing  the  heart  and 
great  blood-vessels  as  well  as  the  lungs.     It  may,  therefore, 


138  ANIMAL   PHYSIOLOGY. 

be  expected  to  exercise  a  certain  influence  on  the  arteriefc 
and  veins,  drawing  in  and  pressing  out  the  blood  in  them,  in 
the  same  way  as  the  air  is  drawn  in  and  pressed  out  through 
the  windpipe;  and  this  is  actually  the  case,  so  far  as  the 
veins  are  concerned,  the  flow  of  blood  into  the  chest  being 
greater  in  inspiration;  but  the  blood  in  the  arteries  is  pro- 
tected from  the  effects  of  inspiration  by  the  strength  of  the 
arterial  coats,  and  thus  inspiration  assists  the  circulation  by 
its  effect  on  the  veins,  without  retarding  the  blood  in  the 
arteries.  The  respiratory  rhythm  consists  of  three  parts — 
the  inspiration,  the  expiration  (which  has  only  half  the 
duration  of  the  inspiration),  and  a  period  of  rest  after  the 
expiration;  and  it  has  been  found  that  the  arterial  pressure 
begins  to  rise  when  inspiration  is  begun,  and  increases  during 
expiration,  then  falls  in  the  period  of  rest  (Sanderson). 

101.  The  enlargement  of  the  thoracic  cavity  in  inspiration 
is  accomplished  partly  by  the  diaphragm,  partly  by  movement 
of  the  ribs.  The  diaphragm  is  the  floor  of  the  thoracic 
cavity,  separating  it  from  the  abdomen.  It  is  a  muscle, 
tendinous  in  the  centre,  and  with  muscular  fibres  radiating 
all  round,  and  inserted  into  lumbar  vertebrae,  into  the  car- 
tilages of  the  lower  six  ribs,  and  into  the  lower  end  of  the 
breast  bone.  When,  at  rest  it  is  arched  upwards,  in  a  dome 
over  the  liver  and  stomach,  and  lies  against  the  costal  walls 
of  the  thorax  for  some  distance  behind  and  at  the  sides ;  but 
when  it  is  contracted,  its  arched  fibres  are  straightened 
and  drawn  asunder  from  the  thoracic  walls,  pressing  down- 
wards the  viscera  beneath  it,  and  producing  a  compensatory 
heaving  of  the  abdomen.  Diaphragmatic  or  abdominal  breath- 
ing occurs  in  a  very  marked  degree  in  children,  the  floor  of 
the  thorax  bearing  a  much  larger  proportion  to  its  height  in 
children  than  in  the  adult. 

The  breathing  of  women  is  sometimes  called  superior  costal, 
and  that  of  men  inferior  costal,  because  the  most  obvious  move- 
ments of  the  ribs  are  above  the  breasts  in  women,  and  in  men 
on  the  sides  of  the  chest.  The  elevation  of  the  ribs  will  be  best 
observed  lay  placing  the  hands  on  the  sides  and  front  of  the 
chest  of  another  person  while  he  inhales  deeply.  The  hands 
will  then  be  felt  to  be  raised  and  separated  one  from  the 
other.  If  the  breast  bone  be  watched,  it  will  be  seen  that  its 


MECHANISM   OF   RESPIRATION. 


139 


rise  and  fall  are  comparatively  slight,  that  only  in  deep  inspira- 
tions is  the  upper  end  of  the  bone  raised  at  all,  and  that 
while  in  the  best  formed  chests  its  lower  end  moves  some- 
what upwards  and  forwards  in  ordinary  breathing,  there  are 
many  chests  in  which  there  is  little  or  no  movement  of  the 


Fig.  78. — VERTICAL  TRANSVERSE  SECTION  OF  CHEST  AND  STOMACH, 
to  show  the  arched  form  of  a,  a,  the  diaphragm;  b,  the  heart;  c,  c, 
lungs;  d,  liver;  e,  stomach;  /,  spleen;  g,  pancreas.  The  reflexions 
of  the  pericardium  and  pleurse  round  the  heart  and  lungs  are 
represented.  Altered  from  Luschke. 

breast  bone  at  all.  The  enlargement  of  the  chest,  by  move- 
ment of  the  ribs,  in  quiet  breathing,  is  therefore  principally 
effected  by  raising  the  lateral  arches  or  hoops  formed  by 
large  ribs,  so  as  to  bring  them  into  the  positions  previously 
occupied  by  smaller  ribs  above  them ;  and  in  deep  inspira- 


140  ANIMAL    PHYSIOLOGY. 

tion  these  are  further  elevated  by  the  first  pair  of  ribs  being 
themselves  raised  by  muscles  descending  from  the  neck.  The 
elevation  of  the  first  pair  of  ribs  increases  the  capacity  of  the 
thorax  considerably;  for  although  those  ribs  are  short,  and 
embrace  but  a  small  portion  of  lung,  their  elevation  involves 
the  additional  raising  of  those  below. 

If  a  finger  be  placed  on  the  twelfth  rib  during  the  fullest 
inspiration,  it  will  be  found  that  it  is  scarcely,  if  at  all, 
raised,  that  its  principal  movement  is  backwards;  and  the 
importance  of  this  will  be  appreciated  when  it  is  considered 
that  the  thoracic  cavity  is  increased  in  size  by  depression  of 
its  floor,  and  would  be  diminished  rather  than  enlarged  by 
elevation  of  the  attachments  of  the  diaphragm.  In  fact,  the 
the  four  lowest  pairs  of  ribs  are  specially  acted  on  in  inspira- 
tion by  a  pair  of  muscles  (serrati  postici  inferiores),  whose 
office  is  to  pull  them  backwards  and  resist  their  elevation. 
The  backward  enlargement  of  the  thorax,  particularly  in  its 
lower  part,  is  further  provided  for  by  the  downward  and 
backward  direction  taken  by  the  ribs  as  they  extend  out 
from  the  back  bone,  and  is  exceedingly  important,  as  it  is  at 
the  lower  and  back  part  of  the  thorax  that  the  greatest  mass 
of  the  lungs  is  situated.  That  is  a  fact  which  ought  to  be 
generally  borne  in  mind,  not  only  as  pointing  out  one  source 
of  evil  from  the  vicious  habit  of  tight-lacing,  but  because  it  is 
a  popular  error  that  in  protecting  the  chest  from  cold,  it  is 
sufficient  to  accumulate  warmth  on  the  fore  part.  Bronchitis, 
no  doubt,  is  liable  to  have  its  source  in  exposure  to  cold 
by  the  mouth  or  in  front  of  the  chest,  but  pneumonia  or 
inflammation  of  the  texture  of  the  lung  is  more  frequently 
traceable  to  exposure  of  the  waist;  for  example,  by  the 
evaporation  of  a  shirt  damp  from  perspiration. 

102.  The  muscles  by  which  the  ribs  are  moved  are  placed 
between  them,  forming  two  layers,  sloped  in  opposite  direc- 
tions, and  named  the  external  and  internal  intercostal 
muscles.*  In  full  inspiration  muscles  of  the  neck  are  also 

*  On  the  Continent  it  seems  to  be  now  generally  admitted  that 
both  these  sets  of  fibres  are  of  service  in  inspiration ;  but  in  English 
text-books  another  theory  continues  to  hold  its  ground,  which  is  only 
thus  referred  to  that  the  student  may  be  warned  against  it  as  an 
error.  The  merits  of  the  question  cannot  be  here  discussed. 


VITAL    CAPACITY.  141 

called  into  action  to  elevate  the  breast  bone  and  first  pair 
of  ribs;  and  when  breathing  is  difficult,  particularly  from 
asthma,  additional  force  is  obtained  by  fixing  the  arms,  as,  for 
example,  by  laying  hold  of  an  arm-chair ;  and  these  muscles 
extending  between  the  chest  and  shoulders,  being  fixed  at 
the  latter  attachment,  exercise  their  action  on  the  former. 
It  is  principally  in  inspiration  that  muscular  force  is  called 
into  requisition,  expiration  being  largely  aided  by  the  elas- 
ticity of  both  ribs  and  lungs ;  for  the  ribs  in  inspiration  are 
pulled  forcibly  out  of  their  natural  curve,  and  the  elasticity 
of  the  pulmonary  texture  may  be  easily  demonstrated  by 
noting  the  rapidity  with  which  the  lungs  of  dead  animals 
collapse  after  artificial  inflation.  In  certain  circumstances, 
however,  a  considerable  exertion  may  be  required  in  expira- 
tion, as,  for  instance,  in  playing  a  wind  instrument  or  in 
glass-blowing ;  and  then  the  abdominal  muscles  may  bo 
brought  strongly  into  action  both  to  push  up  the  diaphragm 
and  to  depress  the  ribs. 

103.  The  acts  of  inspiration  take  place  at  a  rate  usually  vary- 
ing from  fifteen  to  twenty  per  minute ;  but,  like  the  pulsations 
of  the  heart,  they  are  much  more  frequent  in  childhood,  being 
above  forty  per  minute  in  the  infant.     Only  a  small  portion 
of  the  air  in  the  lungs  is  changed  in  each  ordinary  respira- 
tion; the  quantity  so  changed,  or  the  breathing  air,  as  it  is 
termed,  being  on  an  average  twenty  or  twenty-five  cubic 
inches,  while  the  vital  capacity,  or  amount  of  air  which  can 
be  expelled  after   a  full    inspiration,   and  which  therefore 
includes  a  complemental  supply  not  usually  taken  in,  and  a 
reserve  quantity  not  iisually  parted  with,  is  estimated  on  an 
average  at  225  cubic  inches.     Even  after  the  most  forcible 
expiration,  a  large  amount  of  residual  air  remains  in  the 
chest;  and  indeed  it  is  impossible,  even  by  direct  pressure, 
completely  to   expel  the  air  from  the  healthy  lung  of  an 
animal  which  has  breathed,  so  that  it  shall  sink  in  water, 
like  the  lung  of  an  animal  born  dead,  into  which  the  air  has 
never  entered. 

104.  The  vital  capacity  indicates  the  mobility  of  the  walls 
of  the  chest,  but  by  no  means  varies  according  to  the  dimen- 
sions of  the  cavity;  for  differences  in  the  dimensions  occur, 
irrespective   of  height,  age,  or  weight,  whereas   the  vital 


142       .  ANIMAL   PHYSIOLOGY. 

capacity  increases  according  to  the  weight  in  persons  of 
less  than  11|-  stones,  and  in  persons  above  that  weight  is 
jsaid  to  diminish  at  the  rate  of  a  cubic  inch  per  pound;  it 
increases  also  with  age  up  to  the  period  from  the  thirtieth  to 
the  thirty-fifth  year,  then  diminishes  1^  inches  every  year ; 
and  it  increases  regularly  with  the  height  of  the  individual. 
This  increase  of  vital  capacity  according  to  height  is  all  the 
more  interesting,  as  it  was  contrary  to  the  expectation  of  the 
observer  who  first  noted  it  (Hutchinson) ;  because,  as  he 
justly  observed,  height  depends  principally  on  length  of 
limb,  and  he  could  not  see  how  that  could  affect  respiration. 
The  student,  however,  will  observe  that  length  of  limb  gives 
increase  of  surface  exposed  to  contact  with  the  atmosphere, 
and  liable  to  be  cooled  thereby,  and  that  the  person  who  has 
the  greatest  amount  of  surface  requires  the  greatest  amount 
of  combustion  in  his  tissues  to  keep  up  the  temperature  of 
the  body,  and  consequently  requires  more  oxygen,  and  gives 
off  more  carbonic  acid  than  others. 

It  has  been  recently  shown  that  a  distinct  influence  is 
exerted  by  climate  on  the  vital  capacity;  it  being  found  that 
in  the  course  of  a  voyage,  the  capacity,  being  measured  in 
the  north  and  south  temperate  zones,  and  in  crossing  the 
equator,  rises  in  the  tropics  and  falls  again  on  reaching 
temperate  latitudes.  This  curious  phenomenon  apparently 
depends  on  the  lungs  containing  less  blood  and  a  greater 
volume  of  air  in  hot  climates,  so  that  they  are  more  compres- 
sible in  expiration.  It  is  an  accompaniment  not  of  increased 
but  of  diminished  respiration  (Rattray). 

The  advantage  of  a  large  chest  may  be  easily  understood; 
for  the  activity  of  respiration  corresponds  with  the  vital 
capacity,  and  not  with  the  thoracic  dimensions;  it  is  regu- 
lated by  conditions  throughout  the  body,  and  not  by  the  size 
of  the  organ;  therefore  the  smaller  the  lungs  the  greater  the 
work  thrown  on  each  portion  of  them,  and  the  greatest  work 
of  all  is  thrown  on  each  portion  when  small  lungs  are  com- 
bined with  great  height.  Drill  masters  are  right  in  teaching 
that  an  erect  figure  is  good  for  the  lungs;  for  the  ribs  are  so 
attached  to  the  vertebral  column,  that  when  the  column  is 
bent  forwards  their  elevation  is  prevented;  and  that  the  full 
expansion  of  the  chest  requires  a  straight  back  may  be  easily 


RESPIRATION.  143 

demonstrated  by  taking  in  as  full  a  breath  as  possible  in  the 
stooping  posture,  and  then  rising  and  throwing  the  shoulders 
back,  when  it  will  be  found  that  an  additional  quantity  of 
air  can  be  inhaled.  The  stooping  posture  renders  part  of  the 
lung  useless,  throws  the  work  on  the  rest,  and  leaves  a  smaller 
amount  of  residual  air  with  which  to  dilute  that  which  is 
inhaled. 

105.  The   atmosphere  which  we  breathe  consists  of  79 
volumes  of  nitrogen  with  21  of  oxygen,  containing  in  every 
10,000  volumes  four  or  five  of  carbonic  acid.      It  further 
contains  minute  quantities  of  accidental  gaseous  impurities, 
and  has  constantly  numbers  of  exceedingly  small  particles 
of  organic  matter  floating  about  in  it,  including  germs  of 
different  kinds  of  mould.     The  degree  of  moisture  varies  at 
different  times,   but    the    amount  which   it   is    capable    of 
dissolving  increases  rapidly  with  rise  of  temperature. 

The  air  exhaled  in  expiration  differs  from  what  has  been 
breathed,  in  temperature,  moisture,  and  the  quantity  of 
carbonic  acid  and  oxygen  which  it  contains. 

The  temperature  of  the  exhaled  air  is  approached  to  that 
of  the  blood,  varying  usually  from  97°  to  above  99°F.,  accord- 
ing to  the  rapidity  of  the  respiration  and  the  temperature  of 
the  surrounding  air.  When  respiration  is  slow,  the  air  has 
longer  time  to  become  assimilated  in  temperature  to  the 
interior  of  the  body ;  and  when  the  surrounding  temperature  is 
low,  a  longer  contact  is  required  to  approach  it  to  blood  heat. 

The  air  exhaled  is  always  nearly  saturated  with  moisture, 
however  dry  it  may  have  been'when  taken  in;  and  therefore 
the  maximum  of  water  is  removed  from  the  body  by  this 
channel  during  exercise  in  air  which  is  cold  and  dry;  for 
then  the  respiration  is  active,  and  the  air  admitted  and 
warmed  within  the  chest  requires  most  moisture  for  its 
saturation.  The  average  amount  of  water  thus  removed  has 
been  calculated  to  be  from  nine  ounces  to  more  than  a  pound 
in  twenty-four  hours. 

106.  The  volume  of  carbonic  acid  contained  in  the  expired 
air  forms  usually  about  4J  per  cent,  of  its  bulk.    The  propor- 
tion is,  however,  variable.   When  respiration  is  rapid,  the  per- 
centage of  carbonic  acid  in  each  breath  is  diminished,  while 
the  total  amount  exhaled  in  a  given  time  is  increased,  When 


144  ANIMAL    PHYSIOLOGY. 

the  same  air  is  breathed  several  times,  as  happens  in  crowded 
rooms,  and  with  deficient  ventilation,  the  percentage  of 
carbonic  acid  in  the  expired  air  is  increased.  But  this 
accumulation  of  carbonic  acid  in  the  air  furnishes  an  im- 
pediment to  breathing,  independent  of  the  exhaustion  of  the 
oxygen;  for  however  often  the  same  air  be  passed  through 
the  lungs,  it  never  contains  more  than  10  per  cent,  carbonic 
acid.  In  an  artificial  atmosphere,  animals  have  lived  till  the 
percentage  of  carbonic  acid  reached  1 2  and  even  1 8 ;  but 
when  they  are  placed  at  once  in  an  atmosphere  containing 
that  amount,  they  are  immediately  suffocated,  no  matter 
how  great  the  amount  of  oxygen  present.  It  is  plain,  there- 
fore, that  carbonic  acid  inhaled  is  actively  deleterious,  and 
differs  altogether  from  the  nitrogen  contained  in  the  atmo- 
sphere; nitrogen  being  simply  negative,  acting  as  a  diluent 
of  the  oxygen,  incapable  of  taking  the  place  of  that  sub- 
stance, but  in  no  way  interfering  with  its  action.  Animals 
live  without  discomfort  in  an  atmosphere  in.  which  hydrogen 
is  substituted  for  nitrogen. 

The  amount  of  carbonic  acid  exhaled  in  a  given  time  goes 
on  increasing  in  males  till  thirty  years  of  age,  while  from 
forty,  or  sooner,  it  diminishes  as  age  advances. 

In  females  the  amount  ceases  to  increase  at  puberty,  and 
remains  stationary  till  the  cessation  of  reproductive  activity, 
when  it  again  increases  for  a  time.  The  amount  of  carbonic 
acid  exhaled  from  the  lungs  of  an  adult  man  may  be  esti- 
mated as  sufficient  to  yield  about  nine  ounces  avoirdupois  of 
carbon  in  twenty-four  hours.*  But  it  varies  according  to 
a  great  variety  of  circumstances,  being  increased  by  cold,  by 
food,  and  most  of  all  by  exercise ;  while  warmth,  fasting3  rest, 
and  sleep  dimmish  it. 

107.  The  amount  of  oxygen  taken  into  the  blood  in  respira- 
tion does  not  bear  any  constant  proportion  to  the  amount  of 
carbonic  acid  given  off;  but  it  is  generally  somewhat  greater, 
and  is  always  so  when  the  period  of  observation  is  extended 
over  twenty-four  hours.  Carbonic  acid  contains  exactly  its 
own  volume  of  oxygen ;  and  therefore  if  the  oxygen  taken  in 
corresponded  with  the  carbonic  acid  given  off  in  each  respira- 

*  Eight  ounces  is  the  amount  generally  mentioned  in  text-books  ; 
but  that  means  troy  ounces  formerly  in  use  in  matters  medical. 


RESPIRATION.  145 

tion,  there  would  be  just  sufficient  oxygen  to  account  for  the 
formation  of  the  carbonic  acid.  But  there  is  an  additional 
amount  of  oxygen  inhaled,  rendering  the  volume  of  air 
expired  smaller  than  that  which  is  inspired;  and  this  addi- 
tional amount  must  be  used  for  some  other  purpose  than  the 
formation  of  the  carbonic  acid  escaping  by  the  lungs.  A 
small  portion  may  be  used  in  the  formation  of  the  carbonic 
acid  which  escapes  by  the  skin,  estimated  at  one -fiftieth 
of  what  is  exhaled  by  the  lungs;  but  experiments  on  the 
total  respiration,  both  pulmonary  and  cutaneous,  made  by 
placing  a  man  in  an  air-tight  chamber  and  estimating 
the  carbonic  acid  evolved,  agree  with  those  confined  to 
the  pulmonary  in.  showing  that  the  oxygen  given  off  in 
twenty-four  hours,  in  form  of  carbonic  acid,  is  less  than  what 
is  taken  up;  and  we  must  therefore  suppose  that  the  excess 
of  the  oxygen  is  used  in  other  processes  of  oxidation,  con- 
verting the  hydrogen  of  organic  matters  into  water,  and 
their  sulphur  and  phosphorus  into  sulphuric  and  phosphoric 
acids.  This  is  in  keeping  with  the  observation  that  the 
proportion  of  oxygen  absorbed  is  greater  in  feeding  on 
animal  than  on  vegetable  food;  for  the  carbohydrates,  it 
will  be  recollected,  already  contain  as  much  oxygen  as  would 
combine  with  their  hydrogen  to  form  water,  whereas  oils  and 
nitrogenous  substances  are  comparatively  deficient  in  oxygen. 

108.  Gases  brought  into  contact  one  with  another  tend  to 
diffuse  till  they  form  a  uniform   mixture;  and  when  two 
gases  are  separated  by  a  membrane,  they  pass  in  opposite 
directions  through  it  in  definite  proportions.     The  first  of 
these  laws  is,  in  all  probability,  of  the  utmost  importance  in 
diffusing  the  inspired  air  through  the  residual  and  reserve 
air  left  in  the  lungs  after   the  last  expiration.     But  the 
variability  which  has  been  mentioned  in  the  proportion  of 
the  inspired  oxygen  to  the  expired  carbonic  acid,   affords 
sufficient  proof  that  it  is  not  by  diffusion,  as  has  sometimes 
been  supposed,   that  the  interchange  of  these  gases  takes 
place  between  the  air  and  the  blood.     Another  objection  to 
the   supposition  is  that  the  gases  appear  to  be,  at  least  in 
part,  in  chemical  combination  with  the  blood. 

109.  When  respiration  is  obstructed,  either  mechanically  or 
by  deficiency  or   impurity  of  air,  asphyxia   or   suffocation 

U  K 


146  ANIMAL   PHYSIOLOGY. 

takes  place.  The  face  becomes  livid  with  unaerated  blood, 
the  veins  of  the  neck  swollen,  the  circulation  in  the  lungs 
stopped,  and  the  heart  gorged  with  dark  blood,  especially 
on  the  right  side  :  there  is  evidence  that  the  systemic  as 
well  as  the  pulmonary  capillaries  refuse  to  allow  the  blood  to 
pass  through  them  (Dalton),  and  death  quickly  supervenes, 

Even  a  slight  interference  with  respiration  retards  the 
circulation,  and  this  interference  may  be  caused  by  impeded 
expiration,  as  in  blowing  a  trumpet,  or  violent  spasmodic 
expiration,  as  in  coughing,  in  both  which  cases  the  veins  of 
the  neck  are  seen  to  swell ;  or,  by  impeded  inspiration,  as  in 
asthma,  or  by  inhalation  of  a  poisoned  atmosphere,  in  which 
cases  the  hindrance  to  circulation  is  the  vitiated  condition  of 
the  blood. 

The  cessation  of  circulation,  however,  is  not  the  cause  of 
death  by  asphyxia;  that  is  rather  to  be  imputed  to  the 
poisonous  influence  of  vitiated  blood  on  the  brain.  Arrest 
of  the  heart's  action,  constituting  the  condition  known  as 
syncope  or  fainting,  may  be  recovered  from  after  a  period  of 
time  to  which  it  is  difficult  to  name  a  limit ;  but  asphyxia 
causes  death  in  less  than  five  minutes,  and  even  more 
speedily  in  drowning,  which  is  complicated  by  entrance  of 
water  into  the  lungs.  The  few  recorded  instances  of  recovery 
after  submergence  for  longer  periods  are  to  be  accounted  for 
by  supposing  that  the  patient  fainted  at  the  moment  of  falling 
into  the  water,  or  before  falling  in,  and  so  had  lain  without 
effort  at  inspiration. 

110.  It  is  of  importance  to  observe  that  air  may  be 
vitiated  by  many  impurities  besides  carbonic  acid.  Prin- 
cipal among  these  are  minute  particles  of  living  and 
dead  organic  matters  floating  in  the  air,  and  products  of 
decomposition  of  organic  debris.  The  precise  nature  and 
properties  of  the  different  substances  with  which  the  air  may 
be  filled  are  difficult  to  determine.  It  must  not  be  supposed 
that  things  which  are  offensive  to  the  senses  are  necessarily 
deleterious  to  the  health.  There  is  no  proved  relationship 
between  the  intensity  of  bad  smells  and  insalubrity  of  the  air, 
and  many  invectives  about  poisoned  atmospheres,  sufficiently 
excellent  in  their  general  tendency,  are  based  on  very  slender 
scientific  foundation.  But  while  it  is  admitted  that  un- 


VENTILATION.  147 

pleasant  odours  are  not  always  injurious,  there  can  be  no  doubt 
that  constant  exposure  to  the  emanations  of  putrefaction, 
especially  such  as  is  fed  with  meagre  supplies  of  oxygen, 
engendering  products  of  unstable  equilibrium,  is  thoroughly 
baneful,  and  may  exert  its  noxious  influence  with  but  little 
warning  given  to  the  nostrils. 

All  disinfectants  in  use  act  in  one  or  other  of  two  ways : 
they  either  decompose  organic  matter,  or  they  preserve  or 
pickle  it;  permanganate  of  potash,  chlorine,  and  fumes  of 
burning  sulphur  being  examples  of  the  destructive  kind, 
while  creosote  and  carbolic  acid  are  instances  of  the  pre- 
servative or  antiseptic  description. 

111.  Ventilation  has  for  its  object  the  preservation,  within 
buildings,  of  an  atmosphere  as  free  as  possible  from  accumu- 
lation of  carbonic  acid,  or  any  other  impurity,  by  affording 
ingress  to  fresh  air  and  egress  to  the  vitiated.  Practically,  the 
great  problem  is  how  to  attain  this  end  with  as  little  admis- 
sion of  cold  as  possible,  and  without  draughts.  Draughts  are 
not  only  highly  dangerous, '  on  account  of  the  well  known, 
but  ill  understood,  sympathy  between  the  secretion  of 
different  parts  of  the  integument  and  various  internal  organs, 
but  are  deservedly  regarded  with  much  dislike,  a  dislike 
which  may  be  so  great  that  impure  air  will  be  endured  in 
preference.  To  avoid  draughts,  the  communication  of  a  heated 
room  with  the  external  air  should  be  constant,  free,  and  directed 
away  from  the  position  of  the  inmates,  or  the  air  should  be 
heated  by  some  special  contrivance  before  it  gains  admission. 
The  dense  air  from  without  rushes  into  an  apartment  with  the 
greater  force  the  narrower  the  aperture  of  entrance;  and  no 
arrangement  can  well  be  imagined  more  likely  to  produce 
exposure  to  draughts  than  a  room  with  a  warm  fire  on  one 
side,  and  insufficient  ventilation  taking  place  through  the 
key-holes  and  chinks  of  windows  and  doors  on  the  other. 
The  heated  air  passes  up  the  chimney,  and  cold  air  rushes  in 
streams  with  great  velocity  through  the  narrow  apertures 
opposite.  The  density  of  cold  air  gives  it  such  force,  in  rush- 
ing into  a  heated  enclosure,  that  it  travels  inwards  or  falls 
down  through  an  opening  in  the  ceiling  very  compactly ; 
and,  therefore,  in  good  ventilation,  means  should  be  taken  to 
diffuse  the  entering  streams  of  cold  air,  and  direct  them  away 


148  ANIMAL   PHYSIOLOGY. 

from  the  occupants;  and  in  large  nails  the  ventilation  should 
be  placed  at  a  variety  of  levels. 

112.  Internal  Temperature. — The  temperature  of  the  in- 
terior of  the  body  is  very  constant,  its  healthy  variation  being 
limited  to  a  range  of  probably  two  degrees.  In  this  respect 
warm  blooded  animals  differ  from  the  cold  blooded ;  for  the 
evolution  of  heat  in  cold  blooded  animals  being  only  sufficient 
to  warm  them  a  very  few  degrees  above  the  surrounding 
medium,  the  temperature  varies  with  that  of  the  medium. 
In  all  instances  the  heat  evolved  is  the  result  of  chemical 
action;  and  the  processes  in  warm  blooded  animals  being  more 
active,  a  greater  amount  of  heat  is  evolved,  while,  reciprocally, 
a  certain  temperature  is  required  for  the  carrying  on.  of  these 
processes  by  the  more  delicately  constituted  elements  of 
texture. 

The  highest  temperature  is  found  in  birds,  in  some  of  which 
it  reaches  1 1 0°  F.  or  more.  The  temperature  of  the  human 
blood  varies  from  100°  to  102°;  and  it  has  been  already 
mentioned  (p.  125)  that  the  blood  is  the  source  of  warmth 
to  the  tissues.  It  has  not  exactly  the  same  temperature 
throughout  the  circulation,  but  exhibits  definite,  though 
slight,  differences  in  different  parts.  It  was  many  years  ago 
observed  that  the  blood  in  the  jugular  vein  of  an  animal  was 
slightly  cooler  than  .that  in  the  corresponding  artery  (John 
Davy),  and  it  was  judged,  naturally,  that  the  blood  was 
warmed  as  it  became  arterialized  in  the  lungs.  The  observa- 
tion was  correct,  and  yet,  curiously  enough,  the  conclusion 
was  wrong ;  for  in  later  observations  thermometers  have  been 
introduced  by  Bernard,  not  merely  into  the  jugular  vein,  but 
into  the  heart  itself,  and  into  the  renal  and  hepatic  veins, 
with  an  interesting  result  which  explains  the  matter  differ- 
ently. The  warmest  blood  in  the  body  is  that  which  has  been 
subjected  to  processes  of  change  in  the  kidneys  and  liver  ; 
the  coldest  blood  is  that  which  returns  from  the  surface  of 
the  body,  where  heat  is  constantly  lost  by  radiation;  the  blood 
entering  the  heart  from  the  inferior  vena  cava,  containing 
what  comes  from  the  liver  and  kidneys,  is  warmer  than  that 
which  returns  by  the  jugular  vein  from  the  head;  the 
mingled  contents  of  the  right  side  of  the  heart  have  an  inter- 
mediate temperature  ;  this  blood  then  passes  into  the  lungs, 


INTERNAL   TEMPERATURE.  149 

where  it  is  exposed  to  the  inhaled  air,  and  when  it  reaches  the 
arteries  it  is  slightly  colder  than  it  was  when  in  the  right 
side  of  the  heart,  although  it  is  not  quite  so  cold  as  the  blood 
in  the  jugular  vein.  That  the  blood  should  be  cooled  in  passing 
through  the  lungs  is  contrary  to  all  old  beliefs,  but  it  will 
not  strike  the  student  as  strange  when  he  considers  how  much 
heat  is  abstracted  by  the  inhaled  air  before  it  quits  the  lungs. 
The  absorption  of  oxygen  by  venous  blood  is  proved  experi- 
mentally to  be  accompanied  with  a  certain  evolution  of  heat ; 
but  the  quantity  is  not  sufficient  to  balance  the  loss  by  ex- 
posure to  air  inhaled  at  ordinary  temperatures. 

In  disease,  the  temperature  of  the  body  may  vary  greatly 
from  the  healthy  standard ;  in  febrile  affections  it  may  rise 
to  106°  or  more,  and  in  conditions  of  great  feebleness,  such 
as  the  collapse  in  cholera,  it  has  been  known  to  descend 
below  70°. 

It  will  be  understood,  however,  that  the  extremes  of  ex- 
ternal temperature,  which  can  be  borne  with  impunity,  are 
not  accompanied  with  any  such  changes  within  the  body,  but 
illustrate  the  power  which  the  body  has  of  maintaining  its 
own  proper  temperature.  Thus,  in  extreme  cold,  the  greater 
combustion  necessary  in  the  tissues  is  testified  by  the  more 
active  respiration;  while  in  exposure  to  heat,  the  body  is  kept 
cool  by  evaporation.  Temperatures  far  above  what  would 
be  sufficient  to  boil  the  juices  of  the  body,  were  they  exposed 
directly  to  the  heat,  can  be  borne  for  a  short  time  with 
impunity,  provided  always  that  the  air  be  dry,  so  as  to  aid 
free  evaporation  from  the  surface ;  but  moist  air  cannot  be 
endured  above  a  very  moderate  heat. 


CHAPTER  XI. 
ABSORPTION. 

113.  WE  have  now  to  take  into  consideration  the  means  by 
•which  the  substance  of  the  blood  is  replenished.  This  is 
effected  by  absorption,  or  the  sucking  up  of  material  into 
vessels,  partly  from  the  alimentary  canal,  and  partly  from  the 
tissues.  Matter  from  both  these  sources  is  absorbed  by  the 
capillary  blood-vessels,  and  so  carried  into  the  veins;  but 
there  is  another  set  of  vessels,  the  lymphatics,  more  especially 
referred  to  when  absorbents  are  spoken  of,  whose  whole  office 
is  one  of  absorption. 

The  lymphatics  or  absorbents  are  a  system  of  vessels 
with  delicate  walls,  and  having  the  appearance  of  long  and 
slender  threads  when  they  are  empty.  The  trunk  into  which 
the  majority  of  them  pour  their  contents,  the  thoracic  duct, 
is  no  greater  in  diameter  than  a  small  crow  quill,  and  some- 
times not  so  large.  The  thoracic  duct  begins  in  the  upper 
part  of  the  back  of  the  abdomen,  where  it  forms  a  dilatation 
four  or  five  times  its  width  in  the  rest  of  its  course,  named 
the  receptaculum  chyli,  and  runs  up  through  the  thorax  in 
front  of  the  vertebral  column,  to  open,  at  the  root  of  the  neck, 
into  the  angle  of  junction  of  the  left  jugular  and  left  sub- 
clavian  veins.  It  receives  the  absorbents  from  the  whole 
body,  with  the  exception  of  the  right  half  of  the  thorax, 
right  arm,  and  right  side  of  the  head  and  neck.  The 
absorbents  from  these  parts  unite  to  form  a  short  trunk, 
which  opens  into  the  angle  of  junction  of  the  right  jugular 
and  subclavian  veins,  and  is  called  the  right  lymphatic  duct. 

The  lymphatic  vessels  are  difficult  to  study  on  account  of 
their  slenderness,  and  because  they  are  thickly  beset  with 
valves  like  those  of  veins,  which,  in  most  instances,  effectually 


ABSOKBENT   SYSTEM. 


151 


Fig.  79. — ABSORBENT  SYSTEM  :  Diagrammatic  view  of  Lacteals  and 
Lymphatics,  a,  Thoracic  duct  opening  into  the  angle  of  junction 
of  left  subclavian  and  internal  jugular  veins  ;  b,  right  lymphatic 
duct ;  c,  c,  portion  of  small  intestine,  with  lacteals  proceeding 
from  it  to  the  receptaculum  chyli.  On  the  right  arm  the  super- 
ficial lymphatics  are  exhibited ;  on  the  left  the  deep  lymphatics. 


152 


ANIMAL   PHYSIOLOGY. 


prevent  the  injection  of  fluid  backwards  from  the  larger 
trunks  into  the  radicles.  These  valves  are  set  so  thickly  as 
to  give  to  the  vessels,  when  filled,  a 
beaded  appearance,  there  being  a  dila- 
tation opposite  each  valvular  pouch. 
In  the  limbs  and  in  the  walls  of  the 
trunk,  they  are  arranged  in  a  deep  and 
superficial  set;  and  in  the  viscera 
there  is  usually,  in  like  manner,  a  set 
on  the  surface  of  an  organ,  and  a  deep 
set  accompanying  its  blood-vessels. 

At  different  points  in  their  course 
the  lymphatics  are  interrupted  by 
lymphatic  glands,  tough  structures, 
often  about  the  size  of  an  almond,  and 
mostly  arranged  in  groups.  Each  of 
these  receives  a  number  of  lymphatics 
distinguished  as  afferent  vessels,  which 
pour  their  contents  into  it,  and  gives 
off  a  number  of  efferent  vessels  which 
carry  the  contents  onwards.  They 
are  liable  to  be  swollen  or  inflamed  by 
the  irritation  of  fluids  brought  to  them 
from  inflamed  parts,  and  in  that  state 
are  often  felt  through  the  skin  as  hard 
knots,  popularly  known  as  kernels. 
Thus,  hardened  kernels  are  often  felt 
in  the  neck  after  eruptions  on  the 
head,  and  below  and  behind  the  jaw 
after  toothache;  in  the  upper  part  of 
the  thigh,  from  blisters  of  the  foot; 
and  in  the  armpit  from  irritations  on 
Fig.  80.  —  LYMPHATIC  the  arm,  back,  or  breast. 
GLAND  of  the  groin.  m  The  ivmphatics  commence  in 
a,  An  afferent  duct,  n  ,..,,.,., 

with  the  concavities  ver7  fine  networks,  and  in  interstitial 
of  the  valves  turned  spaces  in  the  tissues;  in  some  instances 
toward  the  gland ;  b,  lined  with  exceedingly  delicate  epi- 
b  efferent  ducts  with  thelium,  like  the  capillary  blood-vessels, 
the  convexities  ot  i  •  ,1  .  mi  ^  •  i  i  •  -i 

their  valves  toward  and  m  others  not-  The  flmd  whlch 
the  gland  (Mascagni).  they  contain  is  termed  lymph.  The 


THE   LYMPHATICS.  153 

lymph  is  a  clear,  transparent  fluid,  slightly  alkaline  in 
reaction,  as  is  the  blood,  and  containing  albumen,  salts, 
and  a  variable  amount  of  extractive.  As  it  comes  from 
the  tissues  it  is  perfectly  structureless;  but,  after  passing 
through  lymphatic  glands,  it  contains  lymph  corpuscles 
identical  in  all  respects  with  the  white  corpuscles  of  the 
blood,  and  also  becomes  spontaneously  coagulable,  forming  a 
weak  clot.  The  function  of  the  lymphatic  glands  appears, 
therefore,  to  be  to  form  white  corpuscles;  and  this  view  is 
corroborated  by  their  structure :  for  the  lymphatics,  on  enter- 
ing them,  lose  their  proper  walls,  and  are  continued  into 
irregular  spaces,  winding  between  masses  of  stroma  loaded 
with  corpuscles  which,  as  they  develop,  are  loosened,  and 
float  away  in  the  lymph.  The  addition  of  the  corpuscles 
to  the  lymph  is  sufficient  to  account  for  its  acquiring  the 
property  of  coagulability;  the  reason  for  the  lymph,  as  it 
comes  from  the  tissues,  not  being  spotaneously  coagulable, 
being  simply  that,  like  liquor  sanguinis  drawn  pure  from 
the  vein  (p.  110),  it  contains  no  flbrinoplastin. 

115.  The  absorbents  which  come  from,  the  small  intestine, 
although  they  in  no  way  differ  from  the  lymphatics  of  the 
rest  of  the  body,  are  distinguished  as  lacteals.  The  dis- 
tinction is  unscientific,  inasmuch  as  the  lacteals  are  simply 
the  lymphatics  of  the  small  intestine,  carrying  lymph  and 
nothing  else  when  the  intestine  is  empty ;  but  the  name 
arose  naturally  enough  from  the  milky  appearance  of  their 
contents  during  digestion,  and  must  be  submitted  to.  The 
lacteals  pass  from  the  intestine  back  between  the  folds  of 
the  mesentery  to  reach  the  receptaculum  chyli,  and  in 
their  course  traverse  a  plentiful  group  of  lymphatic  glands, 
named,  from  their  position,  mesenteric,  and  subject,  like 
other  lymphatic  glands,  to  be  irritated  into  inflammation 
and  disease,  when  the  fluids  reaching  them  by  their  afferent 
vessels  are  altered  by  inflammation  of  the  tissues  from  which 
they  are  derived.  Moreover,  such  disease,  unfortunately 
common  in  weak  children,  is  of  much  graver  importance  in 
tlie  instance  of  these  than  of  other  lymphatic  glands,  since 
it  interferes  with  the  passage  of  nourishment  from  the  intes- 
tine into  the  blood.  This  constitutes  the  essence  of  the 
disease  called  tabes  mesenterica. 


154 


ANIMAL   PHYSIOLOGY. 


116.  The  fluid  taken  up  by  the  lacteals  from  the  contents  of 
the  intestine  is  termed  chyle.  The  chyle  resembles  lymph  in 
containing  albumen  and  salts,  and  in  not  containing  nucleated 
corpuscles,  nor  being  spontaneously  coagulable,  until  it  has 
passed  through  lymphatic  glands  ;  but  it  is  distinguished  by 
its  milky  appearance.  This  milkiness  varies  according  to 
the  nature  of  the  diet,  the  greatest  opacity  and  whiteness 
occurring  when  the  food  is  rich  in  oleaginous  constituents; 
and  on  microscopic  examination  it  is  seen  to  depend  on  the 
presence  of  multitudes  of  exceedingly  minute  molecules  of 
oil,  which,  floating,  as  they  do,  in  an  albuminous  fluid,  have 
no  tendency  to  run  together  into  larger  globules.  The  term 
chyle  is  sometimes  applied  not  only  to  the  milky  contents  of 
the  lacteals,  but  also  to  the  emulsion  contained  in  the  intes- 
tine below  the  entrance  of  the  pan- 
creatic duct,  which  is  the  source  from 
which  the  lacteals  are  filled;  and  a 
sharp  distinction  is  thus  drawn 
between  the  chyme  or  pulp  formed 
by  the  action  of  the  stomach,  and 
the  material  lower  in  the  intestine. 
This  extended  use  of  the  term  chyle 
is,  however,  objectionable,  as  the 
two  fluids  to  which  it  is  applied  are 
not  identical;  that  which  is  found 
in  the  intestine  containing  not  merely 
what  is  fitted  to  be  taken  into  the 
lacteals,  but  large  quantities  of  other 
matters,  partly  destined  to  be  ab- 
sorbed by  the  capillary  blood-vessels, 
Firr,  81.— Two  VILLI  of  anc^  Par%"  to  pass  onwards  and  under- 
human  intestine  de-  go  further  change,  or  be  ejected, 
prived  of  their  epi-  117.  The  nourishment  prepared 
thelium,  and  with  the  jn  the  alimentary  canal  is  taken  up, 

BaeLteSecTed.bThe'Iae::  P^7  ***    *"   blood-vessels,    and 

teals     are    white,    the  partly  into  the  absorbents;  and  al- 

blood  -  vessels      black  though  this  process  undoubtedly  may 

(Teichmann).    ,  &  g0  on  ^o  some  extent  in  the  stomach 

and  whole  length  of  the  intestinal  tract,   it  is  carried  on 

most  actively  by  the  villi  of  the  small  intestine  (p.  99). 


INTESTINAL   ABSORPTION. 


155 


Every  villus  presents  on  its  surface  a  coat  of  columnar 
epithelium  continuous  with  that  of  the  rest  of  the  mucous 
membrane,  and  beneath  this  a  network  of 
capillary  blood-vessels,  while  in  the  centre 
there  is  a  lacteal.  In  the  lower  animals 
there  are  sometimes  several  lacteals  forming 
loops  in  one  villus;  but  in  the  human  sub- 
ject there  is  usually  only  one  in  each. 
This  lacteal  is  somewhat  dilated  at  its  ex- 
tremity, and  it  does  not  appear  at  that  part 
to  have  any  special  wall. 

When  absorption  is  going  on,  the  epi- 
thelial cells  of  the  villi  become  turbid  with 
molecules  of  oil,  the  tissue  beneath  becomes 
turbid  likewise,   and  from  the   tissue   the 
lacteals  are  filled.      The  free  extremity  of 
each    columnar    epithelial    cell  presents   a 
thick  layer  of  substance  less  consistent  than 
the  rest  of  the  cell  wall,  but  more  consistent 
than  its  contents,  and  sometimes  having  a 
vertically   streaked   appearance.      Through 
this  substance  the  molecules  of  emulsified 
oil  find  their  way;  and  afterwards  they  pass 
onwards  through  fine  prolongations  of  the  deep  ends  of  the 
epithelial  cells  :   these  appear  to 
communicate    with    branches    of  a 
connective  -  tissue  -  corpuscles,      by 
which  in  like  manner  the  mole- 
cules are  carried  to  the  lacteal. 

Only  a   small  quantity  of  oil 
is  taken  into  the  capillary  blood- 
vessels, by  far  the  greater  amount 
being  absorbed   by  the  lacteals ;  Z» 
and    this,    indeed,    is    the   most  c 
notable  difference  between  lacteal  j 
and  capillary  absorption,  so  far  as 
nutritive  matters    are  concerned, 
for  it   is   very  certain   that   sac- 
charine and  albuminoid  substances 
are  taken  up  freely  by  both  sets 


82.— COLUM- 
NAR, EPITHELIAL 
CELLS  OF  IN- 
TESTINE, highly 
magnified,  show- 
ing the  striated 
substance  on 
their  free  ex- 
tremities. 


•ig.  83.— BLOOD-VESSELS  OF 
Mucous  MEMBRANE  ov 
STOMACH  :  vertical  section 
magnified,  a.  Venous  radi- 
cles descending  from  the 
free  surface ;  6,  veins ;  c, 
arteries. 


156  ANIMAL  PHYSIOLOGY. 

of  vessels.  The  "arrangement  by  which  the  blood-vessels 
are  enabled  to  furnish  material  for  the  nourishment  of  tex- 
ture, and  formation  of  secretions,  while  they  also  absorb 
matters  presented  to  them  in  both  stomach  and  intestine, 
is  a  very  beautiful  one.  It  is  plain  that  blood,  to  which 
variable  amounts  of  chance  ingredients  had  been  added, 
would  be  unfit  for  purposes  of  nutrition ;  but  the  difficulty 
is  got  over  in  this  way,  that  the  arterioles  break  up  into 
capillaries  in  the  deep  part  of  the  mucous  membrane,  which 
first  supply  the  glandular  structure,  and  then  open  into 
venous  radicles  which  take  origin  close  to  the  surface;  so 
that  the  blood,  in  its  purity,  nourishes  the  glands,  and  does 
not  take  up  foreign  matters  until  about  to  enter  the  veins. 

118.  The  passage  of  liquids  through  membranes  is  regulated 
by  physical  laws  of  diffusion,  which  are  closely  connected 
with  capillary  attraction.  Just  as  gases  diffuse  according 
to  definite  laws,  so  also  do  liquids.  Their  diffusion  through 
membranes  or  porous  septa  is  called  osmosis;  or,  inasmuch  as 
there  are  two  currents  in  opposite  directions  wherever  a 
membrane  separates  two  different  fluids,  the  words  endos- 
mosis  and  exosmosis  may  be  used  to  indicate  the  inward  and 
outward  flow.  If  a  piece  of  moist  bladder  be  stretched 
across  a  tube,  and  any  saline  solution  introduced  into  the 
vessel  thus  made,  and  the  end  of  the  tube  be  then  dipped  in 
water,  it  will  be  found  that  in  a  short  time  a  portion  of  the 
solution  has  passed  through  into  the  water,  while  a  larger 
amount  of  water  has  passed  into  the  tube,  and  raised  the 
height  of  the  liquid  within  it.  The  same  experiment  may 
be  made  with  solutions  of  different  sorts  on  the  two  sides  of 
the  membrane.  But  the  important  points  to  note  are,  that 
different  solutions  pass  through  in  definite  proportion  to  the 
amount  of  any  particular  substance  passing  in  the  opposite 
direction,  and  that  while  some  substances  diffuse. with  facility, 
others  do  so  with  difficulty.  The  substances  which  diffuse 
easily  are  called  crystalloids,  while  those  which  diffuse  with 
difficulty  are  called  colloids  (Graham).  Thus  albumen  in  its 
ordinary  condition  is  a  colloid,  but  when  converted  into  pep- 
tone it  becomes  crystalloid.  It  is  in  consequence  of  endos- 
mosis  that,  when  water  is  added  to  blood,  the  red  corpuscles 
become  swollen  and  spherical.  The  substance  in  which  the 


INTESTINAL  ABSORPTION.  157 

fluid  parts  of  the  corpuscle  are  entangled  acts  as  a  membrane 
would,  and  while  a  certain  amount  of  fluid  passes  out,  a 
larger  amount  of  water  passes  in  and  gorges  the  corpuscle. 
So  also  nucleated  cells,  when  water  is  added  to  them,  become 
rapidly  swollen,  till  they  burst  and  are  destroyed. 

Now,  there  seems  no  reason  to  doubt  that  the  absorption  into 
the  capillary  blood-vessels  is  an  instance  of  endosmosis  with- 
out intervention  of  any  vital  force.  All  sorts  of  salts  and 
other  diffusible  substances,  whether  simply  useless  or  posi- 
tively injurious,  find  their  way  into  them;  while  it  is  proved 
by  experiment  that  such  substances  do  not  pass  into  the  lac- 
teals,  at  least  so  rapidly.  And  this  is  not  altogether  inexplic- 
able; for  we  have  seen  that  the  capillaries  are  near  the  surface 
of  the  villus,  while  the  lacteal  is  in  the  centre,  and  receives 
its  supplies  through  the  action  of  nucleated  corpuscles.  In 
fact,  a  little  reflection  will  show  that  the  action  of  the 
epithelium  in  lacteal  absorption  differs  from  that  of  secreting 
cells  in  separating  substances  from  the  blood  in  nothing, 
save  that  in  secretion  the  current  is  from  vessels  to  a  free  sur- 
face, while  in  lacteal  absorption  the  current  is  from  a  free 
surface  to  the  interior  of  a  vessel. 

119.  The  difference  in  the  absorbing  power  of  the  lacteals 
and  the  blood-vessels  has  probably  a  considerable  importance, 
dependent  on  the  different  courses  taken  by  the  two  sets  of 
vessels.  The  lacteals,  which  are  deeply  placed  in  the  villi, 
and  fed  by  other  means  than  mere  endosmosis,  send  their 
contents,  by  the  thoracic  duct,  into  the  circulation  at  a  point 
where  the  blood  is  returning  to  the  heart  and  has  only  to 
be  subjected  to  the  influence  of  respiration  before  being 
diffused  throughout  the  body.  The  blood-vessels,  on  the 
other  hand,  take  up  everything  according  to  its  diffusibility; 
but  they  carry  their  stream  into  the  portal  vein,  whence  it 
is  conveyed  through  the  capillaries  of  the  liver;  and  not 
until  it  has  been  subjected  to  the  influence  of  that  organ — 
which  has,  besides  other  functions,  an  arrestive  power — is  it 
allowed  to  reach  the  heart. 

As  in  the  case  of  intestinal  absorption,  so  also  throughout 
the  body,  fluids  appear  to  pass  into  the  blood-vessels  easily 
by  endosmosis;  and  the  circumstances  are  not  well  known 
which  call  for  the  necessity  of  lymphatics  as  well  as  blood- 


158  ANIMAL   PHYSIOLOGY. 

vessels.  Yet,  when  it  is  considered  that  lymphatics  at  their 
origin  are  less  distinctly  bounded  than  the  capillary  blood- 
vessels, and  much  more  closely  communicate  with  the  fluids 
of  the  tissues,  it  seems  not  improbable  that,  as  has  been 
suggested,  the  crystalloids  are  principally  taken  into  the 
blood,  while  the  colloids  left  behind  are  carried  away  by  the 
lymphatics. 


CHAPTER  XII. 

THE  DUCTLESS  GLANDS,  THE  LIVER,  AND  THE 
KIDNEYS. 

IN  the  preceding  chapters  we  have  so  far  traced  the  history 
of  the  blood,  that  we  have  seen  how  it  is  vitiated  in  the 
tissues  and  oxygenated  in  the  lungs,  and  how  it  is  replenished 
with  material  both  from  the  waste  of  the  tissues  and  from 
the  digested  food;  we  have  noted  one  source  of  origin  of  the 
corpuscles,  and  studied  the  purification  from  carbonic  acid. 
But  there  still  remain  for  consideration  various  processes  of 
elaboration  and  depuration  carried  on  by  the  spleen  and 
other  ductless  glands,  by  the  liver,  and  by  the  kidneys. 

120.  The  Ductless  Glands. — Under  this  head  are  grouped 
the  spleen,  the  thyroid  and  thymus  glands,  the  suprarenal 
capsules,  and  also  the  lymphatic  glands  and  the  closed 
follicles  of  the  digestive  tube,  both  of  which  have  been 
already  considered  (pp.  103  and  153). 

The  thyroid  body  is  a  limited  structure,  consisting,  in  the 
human  subject,  of  two  lobes  joined  together  by  an  isthmus, 
and  situated  on  the  windpipe  in  the  neck,  and  is  larger  in 
the  female  than  the  male.  It  is  exceedingly  vascular,  and 
consists  of  numbers  of  minute  vesicles,  with  glairy  contents, 
and  each  invested  with  a  rich  network  of  capillaries.  It  is 
sometimes  subject  to  enormous  enlargement,  constituting  the 
disease  called  goitre,  a  tumour  remarkable  not  merely  for 
the  great  size  which  it  sometimes  acquires,  but  for  being 
associated  frequently  with  a  form  of  idiocy  and  general 
deficiency  of  development,  to  which  the  name  of  cretinism  is 
given.  But  nothing  precise  is  known  of  the  function  of  the 
thyroid  body. 

The  thymus  gland  is  a  structure  situated  lower  down  on 
the  windpipe  than  the  thyroid,  being  placed  in  the  upper 


160  ANIMAL  PHYSIOLOGY. 

part  of  the  chest.    In  its  early  history  it  is  closely  associated 
with  the  thyroid,  but  its  structure  is  different  in  detail. 


Fig.  84. — THYROID  AND  THYMUS  BODIES,  three  months  before  birth. 
a,  Thyroid  body  resting  on  trachea  and  larynx ;  6,  6,  thymus ; 
c,  c,  lungs,  as  yet  unexpanded;  d,  heart. 

It  reaches  its  greatest  development  in  infancy,  and  dis- 
appears usually  with  childhood.  Its 
function,  supposed  to  be  connected 
with  blood-elaboration,  is  unknown. 

The  suprarenal  capsules,  are  a  pair 
of  bodies  surmounting  the  kidneys, 
and  fitting  on  to  their  upper  ends. 
Like  the  thymus,  they  are  exceedingly 
large  before  birth.  They  are  highly 
vascular  in  the  interior,  but  are  like- 
wise remarkable  for  their  copious 
supply  of  sympathetic  nerves,  and  it 
Fig.  85.— SUPRARENAL  is  scarcely  safe  to 'say  that  they  are 
CAPSULE  AND  KID-  blood- elaborating  glands  at  all.  All 
NEY,  three  months  that  is  known  about  them  is,  that 
before  birth;  the  kid-  there  ig  a  peculiar  form  of  wasting 
ney  presenting  at  this  -,.  .  T  ..-.  •*  , 

period     a    lobulated  Disease  accompanied  with  deep  bronz- 
surface.  ing  of  the  skin,  in  which  there  is  fre- 


THE   SPLEEN.  161 

quently  found,  after  death,  an  extensive  alteration  of  these 
structures. 

121.  The  spleen  (figs.  57  and  78)  is  an  organ  engaged  beyond 
all  question  in  the  elaboration  of  the  blood,  and  however 
obscure  the  particulars  of  its  function  m'ay  be,  there  is  at 
least  more  known  about  it  than  about  the  other  organs 
which  have  been  briefly  described.  It  is  the  largest  of  the 
ductless  glands,  very  variable  in  size,  but  usually  from  5  to 
7  oz.  in  weight.  It  increases  largely  some  hours  after  eat- 
ing, then  gradually  diminishes  while  fasting  is  continued. 
It  is  a  flattened  oval  body  about  4  or  5  inches  long,  and  3 
inches  or  more  in  breadth,  and  lies  against  the  left  end 
of  the  stomach.  It  has  a  tough  capsule,  and  consists  of  a 
deep  purple  pulp  imbedded  in  the  meshes  of  a  network  of 
fibrous  trabeculse,  which  is  highly  elastic,  and  probably  also 
contains  some  muscular  tissue.  The  pulp  consists  of  granular 
bodies  of  deep  colour  and  about  the  size  of  blood  corpuscles, 
and  nucleated  corpuscles  of  very  variable  size,  the  larger  of 
which  have  several  nuclei.  In  sections  of  spleens  of  the 
domestic  animals,  and  in  spleens  of  young  subjects,  but  not 
so  easily  detected  in  the  healthy  adult  human  spleen,  are 
less  deeply  coloured  spots  like  sago  grains,  called  Malpighian 
corpuscles  of  the  spleen.  They  are  collections  of  small 
nucleated  corpuscles  in  the  sheaths  of  the  arterioles.  The 
splenic  artery  and  vein  are  very  large  for  the  size  of  the 
organ.  The  venous  blood  is  conveyed  into  the  portal  vein 
to  be  sent  through  the  liver. 

The  very  fact  that  the  large  supply  of  arterial  blood  sent 
to  the  spleen  is,  after  passing  through  that  organ,  transmitted 
to  the  liver,  seems  to  point  to  its  having  undergone,  mean- 
while, some  great  change  which  renders  necessary  the  action 
of  the  liver,  as  well  as  of  respiration,  before  it  is  fit  again  to 
traverse  the  tissues,  and  this  idea  is  supported  by  examina- 
tion of  the  blood.  The  blood  in  the  splenic  vein  has  the 
serum  of  a  reddish  colour  unlike  that  of  any  other  blood, 
and  it  contains  less  solid  matter  than  other  venous  blood,  a 
circumstance  easily  explained  on  the  supposition  that  there 
is  in  the  spleen  a  greater  amount  of  chemical  action,  involv- 
ing the  formation  of  carbonic  acid  and  water  at  the  expense 
of  solid  matter,  than  occurs  in  the  tissues  throughout  the 
H  t, 


162  ANIMAL   PHYSIOLOGY. 

*» 

body.  It  contains  also  a  smaller  proportion  of  red  cor- 
puscles than  other  blood,  which  perhaps  results  from  a 
portion  of  their  contents  having  transuded  into  the  serum, 
as  shown  by  its  colour,  and  by  the  corpuscles  being  firmer, 
smaller,  and  more  nearly  spherical.  Lastly,  the  colourless 
corpuscles  are  exceedingly  plentiful;  and  it  is  to  be  noted 
that  in  a  diseased  condition  called  leucocythaemia,  in  which 
the  colourless  corpuscles  of  the  blood  are  remarkably  in- 
creased in  number,  there  is  likewise  great  enlargement  of 
the  spleen.  These  are  the  principal  facts  on  which  rest  the 
two  theories  generally  held  as  to  the  function  of  the  spleen, 
namely,  that  it  is  a  manufacturer  of  white  corpuscles,  and  a 
destroyer  of  the  red. 

That  the  spleen  is  a  source  of  white  corpuscles  can  scarcely 
be  doubted;  but  that  it  is  its  special  function  to  destroy  red 
corpuscles,  is  not  so  clear.  ISTo  doubt  heaps  of  withered  red 
corpuscles  have  been  seen  in  the  spleen  as  an  exceptional 
occurrence;  but  it  is  plain  that  there  is  an  action  exercised 
on  all  the  red  corpuscles,  and  it  seems  very  possible  that  the 
object  of  that  action  is  restoration  rather  than  destruction. 
A  very  curious  circumstance,  which  by  no  means  makes  the 
function  of  the  spleen  more  comprehensible,  is  that  the  whole 
organ  may  be  extirpated,  not  only  without  death  ensuing, 
but  without  any  inconvenience  resulting.  It  is  obvious, 
however,  that  this  is  more  explicable  on  the  supposition  that 
the  spleen  is  one  of  many  structures  which  produce  blood 
corpuscles,  than  if  we  consider  it  as  the  sole  agent  of  their 
destruction. 

122.  The  Liver  (figs.  57  and  78).— -The  liver  is  much  the 
largest  gland  in  the  body,  between  3  and  4  Ibs.  in  weight, 
and  of  remarkable  complexity,  both  of  structure  and  func- 
tion. Its  most  obvious  function  is  to  secrete  bile;  but  the 
bile  is  secreted  less  for  the  sake  of  its  utility  in  digestion, 
than  as  a  product  resulting  from  certain  complicated  pro- 
cesses of  blood-purification. 

The  liver  lies  beneath  the  diaphragm,  with  the  greater 
part  of  its  bulk  on  the  right  side,  under  cover  of  the  ribs, 
and  from  its  position  is  the  organ  most  liable  to  compression 
and  injury  by  tight-lacing.  It  has  a  right  and  left  lobe,  and 
is,  in  fact,  a  bilateral  organ  in  all  vertebrate  animals.  It  even 


THE   LIVEK.  163 

happens  in  a  few  fishes  that  these  lobes  are  quite  discon- 
nected, forming  symmetrically  placed  right  and  left  livers. 
Before  birth  the  liver  bears  a  much  larger  proportion  to  the 
body  than  it  does  afterwards;  and  the  bile  which  is  at  that 
time  secreted  by  it  accumulates  in  the  intestine,  to  be  ex- 
pelled when  the  child  is  born,  and  is  called  meconium. 
Originally  the  liver  occupies  the  whole  of  the  upper  part  of 
the  abdomen,  and  the  left  lobe  is  as  large  as  the  right;  but 
this  does  not  continue  long;  and  in  the  adult,  the  left  lobe  is 
comparatively  small,  and  falls  considerably  short  of  reaching 
the  left  side  of  the  body. 

The  greater  part  of  the  blood  sent  to  the  liver  enters  it  by 
the  portal  vein,  and  comes  from 
the  stomach,  intestine,  and 
spleen;  but  there  is  likewise  a 
hepatic  artery  which  nourishes 
the  textures  of  the  viscus;  and 
the  blood  entering  by  this 
channel  is  afterwards  conveyed 
to  the  capillaries  of  the  portal 
system.  The  hepatic  substance 
is  arranged  in  minute  lobules,^ 
each  of  which  has  ramifications  . 
of  the  portal  vein  in  its  circum- " 
ference,  and  in  its  centre  a 
radicle  of  the  hepatic  vein,  by  Fig.  86.  —  HEPATIC  LOBULE, 
which  the  blood  is  carried  from  capillary  network :  radicle  of 
,i  T  •  ,  j-i  A  hepatic  vein  in  the  centre, 

the  liver  into  the  vena  cava.    A      ang  branclies  of  portai  vein 

rich  network  of  small-meshed  at  the  circumference,  together 
capillaries  not  only  unites  the  with  two  small  twigs  of  hep- 
branches  of  the  portal  and  •  atic  artery, 
hepatic  veins,  but  pervades  the  whole  organ,  passing  con- 
tinuously from  one  lobule  to  another.  Only  a  few  animals, 
such  as  the  pig,  furnish  an  exception  to  this  rule,  and  have 
the  hepatic  lobules  distinct. 

When  the  blood-vessels  are  empty,  a  section  of  liver  under 
the  microscope  exhibits  little  else  than  a  mass  of  nucleated 
corpuscles.  These  corpuscles,  termed  the  hepatic  cells,  are 
somewhat  flattened  polyhedral  bodies  of  an  average  diameter 
of  T^Q  o-  of  an  inch,  of  a  yellowish  tinge,  containing  numerous 


164 


ANIMAL   PHYSIOLOGY. 


salivary    glands 


granules,  and  in  the  human  subject  distinct  oil  globules. 
They  have  a  radiating  columnar  arrangement  in  the  lobules, 

and  pour  their  secre- 
tion into  a  network 
of  minute  intercellu- 
lar channels,  defi- 
nitely walled,  and 
capable  of  being  in- 
jected; which,  how- 
ever, are  not  to  be 
considered  as  proper 
secreting  ducts,  but 
correspond  with  simi- 
lar channels  between 
the  cells  contained 
within  the  saccules  of 
the 
and  pancreas. 

In    the     livers    of 
many        invertebrate 
animals,  the  secreting 
cells  are  obviously  ar- 
Fig.  87.— A,  HEPATIC  STRUCTURE  highly  ranged  in  the  form  of 
magnified;   a    a,  section  of  capillaries ;  an  epithelial  lining  Of 
b,  ft,  intercellular  channels  (Hermg).     B,       ,  •£  ,   ,    ,     -,       -,  % 
Individual  hepatic  cells.  a  lobulated  gland  (fig. 

34),  and  there  can  be 

no  doubt  that  the  hepatic  cells  of  vertebrata  are  morphologi- 
cally comparable  with  these.  An  appearance  of  a  limiting 
membrane  has  even  been  demonstrated  by  one  observer 
(Beale),  surrounding  the  columns  within  the  lobules,  and  it 
is  legitimate  to  look  on  these  columns  as  consisting  of  inter- 
communicating tubules  filled  with  secreting  epithelium.;  but 
they  are  not  so  distinct  as  the  blind  extremities  of  any  other 
gland. 

Minute  bile  ducts,  with  independent  walls,  are  seen  between 
the  lobules;  and  these  fall  into  ducts  of  larger  size,  copiously 
beset  with  mucous  glands.  A  right  and  left  trunk  emerge 
from  the  corresponding  lobes,  and  unite  to  form  the  hepatic 
duct  with  which  the  gall  bladder  communicates  by  a  duct  of 
ibs  own,  the  cystic,  to  form  with  it  the  common  bile  duct. 


LIVER.  165 

The  gall  bladder  is  a  reservoir  in  which  the  bile  is  stored  till 
required  in  the  intestine;  but  although  very  generally  found 
in  all  classes  of  vertebrata,  there  are  some  animals  in  which 
it  is  absent. 

123.  The  Bile  contains  a  large  amount  of  mucus  derived 
from  the  glands  of  the  ducts,  no  doubt  of  some  important 
use;  but  what  that  use  is,  unless  it  be  to  protect  the  mucous 
membrane  of  the  ducts  and  gall  bladder  from  the  action  of 
bile  long  left  in  their  interior,  is  unknown.  It  contains  also 
a  special  colouring  matter,  rich  in  iron,  and  no  doubt  derived 
from  the  colouring  matter  of  the  blood,  so  much  of  which  is 
found  in  the  serum  of  the  splenic  vein.  But  the  most 
characteristic  substances  are  those  which  give  it  the  proper- 
ties of  a  soap,  as  stated  in  a  previous  page  (p.  102).  These 
substances  are  termed  the  glycocholate  and  taurocholate  of 
soda.  Their  acids  are  of  very  complex  composition,  and 
easily  resolved,  the  one  into  cholic  acid  and  glycin,  the  other 
into  cholic  acid  and  taurin.  Cholic  acid  is  a  substance  very 
similar  in  composition  to  the  ordinary  fatty  acids,  having, 
like  them,  a  large  number  of  equivalents  of  carbon  and 
hydrogen  in  combination  with  a  much  smaller  quantity  of 
oxygen;  glycin  and  taurin  are  bases  of  comparatively  simple 
chemical  formula,  and  containing  nitrogen,  while  taurin 
contains  sulphur  in  addition.  It  will  be  noted,  therefore, 
that  the  biliary  acids  must  be  formed,  at  least  in  part,  from 
the  nitrogenous  constituents  of  the  blood.  They  do  not  pre- 
exist in  the  blood,  but  are  manufactured  by  the  liver;  for 
they  are  never  found  even  in  the  smallest  quantity  in  analyses 
of  healthy  blood.  Bile  likewise  contains  phosphates  in 
quantity;  also  a  certain  amount  of  cholesterin,  a  non-nitro- 
genous  crystalline  substance,  allied  to  the  fats,  and  crystal- 
lizing in  large  quadrilateral  plates,  a  constituent  of  the 
brain,  and  very  possibly  brought  to  the  liver  from  that 
source. 

From  experiments  on  animals,  in  which  the  bile  is  gathered 
by  means  of  a  fistulous  opening,  the  flow  would  appear  to 
be  very  great,  and  is  calculated  at  two  or  three  pounds  per 
diem  in  the  human  subject.  It  is  increased  a  few  hours 
after  eating,  and  reaches  its  maximum  about  the  eighth  hour, 
then  gradually  diminishes  while  fasting  is  continued.  The 


166  ANIMAL   PHYSIOLOGY. 

flow  is  augmented  by  flesh,  diet,  but  not  by  a  diet  consisting 
exclusively  of  fat. 

124.  Within  the  substance  of  the  liver  another  product 
besides  bile  is  found,  namely,  glycogen  (Bernard).    This,  which 
is  likewise  called  the  amyloid  substance  of  the  liver,  is  a  material 
similar  to  starch  or  dextrin,  of  the  same  chemical  composition, 
and  easily  converted  into  sugar.     It  is  obtained  as  a  white 
powder  by  precipitation  in  alcohol  from  a  filtered  extract  of 
boiled  and  pounded  liver.     It  is  stored  in  the  hepatic  tissue, 
probably  within  the  cells,  and  after  death  is  speedily  converted 
in  considerable  quantities  into  sugar,  which  is  found  in  the 
blood-vessels  of  the  organ.     For,  if  a  liver  newly  excised 
from  an  animal  have  the  vessels  washed  out  with  water  in- 
jected through  the  portal  vein  till  it  is  quite  free  from  sugar, 
in  a  few  hours  afterwards,  if  they  are  again  washed   out, 
abundance  of  sugar  will  be  obtained.     What  becomes  of  the 
glycogen  during  life,  however,  is  a  point  on  which  some 
difference  of  opinion  exists;  but  it  may  be  mentioned  that 
even  those  who  are  most  sceptical  of  its  passage  during  life 
into  the  blood,  have  themselves  found  sugar  in  at  least  minute 
quantities  in  blood  drawn  from  the  right  side  of  the  heart 
of  living  animals  by  means  of  a  pipette  introduced  by  the 
jugular  vein  (Pavy  and  McDonnell):  and  the  student  will 
recollect  that  it  has  been  already  pointed  out  (p.   109)  that 
the  smallness  of  the  quantity  of  any  substance  in  the  blood 
is  no  proof  that  a  large  amount  of  it  does  not  pass  through 
the  circulation,  but  only  shows  that  it  does  not  accumulate 
there.     The  sugar  which  enters  the  circulation  from  the  liver 
is,  in  health,  destroyed  or  altered  in  the  lungs. 

Analyses  of  the  livers  of  animals  which  had  been  fed  for 
several  days  on  one  particular  diet,  show  that  glycogen  ac- 
cumulates most  rapidly  when  the  diet  consists  of  starch  and 
sugar,  but  that  it  is  also  formed  from  purely, nitrogenous 
food;  while,  when  fat  and  gelatin  alone  are  consumed,  the 
liver  is  free  from  glycogen  (McDonnell).  •% 

125.  The  blood,  in  passing  through  the  liver,  undergoes  a 
marked  amount  of  change.     This  is  known  by  analysis  of 
the  blood  entering  by  the  portal  vein,  and  that  which  leaves 
by  the  hepatic.     The  blood  of  the  hepatic  vein  is  of  very 
dark  colour,  and  its  corpuscles  resemble  those  of  the  splenic 


THE    LIVER.  1G7 

vein  in  resisting  the  action  of  water,  and  collecting  in  heaps 
instead  of  numnmlar  rows.  It  forms  no  spontaneous  coagn- 
lum,  and  only  a  small  quantity  of  fibrin  can  be  obtained 
from  it,  even  when  whipped  with  rods.  It  has  both  a  larger 
proportion  of  corpuscles,  and  a  larger  amount  of  solid  matter 
in  its  serum  than  the  portal  blood;  that  is  to  say,  it  has  less 
water,  as  one  might  expect  from  the  fact  that  the  bile  has  a 
much  lower  specific  gravity  than  blood.  The  Solid  residue 
obtained  by  evaporation  of  its  serum,  contains  a  very  de- 
cidedly smaller  amount  of  albumen  than  the  solid  substance 
of  the  portal  serum,  a  smaller  amount  of  fat,  and  a  large 
increase  in  the  amount  both  of  salts  and  extractive  matters. 
"We  must  conclude,  therefore,  that  albuminoids  are  decom- 
posed in  the  liver,  and  that  their  decomposition  gives  rise  to 
the  nitrogenous  part  of  the  additional  extractive  matters  in 
the  hepatic  vein. 

126.  The  student  has  now  been  put  in  possession  of  the  prin- 
cipal known  facts  which  bear  on  the  question  of  the  use  of 
the  liver  in  the  economy,  and  it  remains  to  consider  what  its 
uses  are.  To  do  so,  is  to  pass  from  the  region  of  fact  to  that 
of  hypothesis,  and  it  is  well  that  the  student  should  under- 
stand how  widely  different  these  are,  one  from  the  other.  It 
is  the  deficiency  in  our  knowledge  of  facts  which  necessitates 
conjecture;  were  the  chain  of  facts  complete,  the  need  for 
conjecture  would  cease. 

Some  points  are  beyond  all  doubt:  the  liver  certainly 
arrests  fats  and  sugar,  storing  the  latter  up  in  the  form  of 
glycogen.  Whether  it  arrests  other  substances  is  not  so 
certain;  but  it  cannot  be  denied  that  after  death  from  arsenic 
and  other  metals,  the  poison  is  found  in  particular  abun- 
dance in  this  viscus.  It  rids  the  blood  of  the  colouring 
matter  which  stains  the  serum  coming  from  the  spleen, 
and  it  decomposes  albuminoids,  either  on  account  of  their 
being  in  a  condition  unfit  for  circulation,  or  in  overabun- 
dance. The  products  of  this  decomposition  are,  it  may  be 
supposed,  nitrogenous  extractive  matters  remaining  in  the 
blood,  and  the  bile  removed  from  it;  and  it  is  quite  possible 
that  glycogen,  when  present  in  the  hepatic  cells,  may  assist 
the  decomposition,  though  it  is  certain  that  bile  is  formed 
when  glycogen  is  absent  from  the  liver.  It  has  been  pre- 


16&  ANIMAL   PHYSIOLOGY. 

viously  pointed  out  (p.  103)  that  the  biliary  acids  are  decom- 
posed in  the  intestine,  and  that  the  products  are  probably 
in  part  absorbed.  The  kind  of  nourishment  resulting  from 
decomposition  of  the  biliary  acids,  and  taken  up  again  from 
the  intestine,  is  not  known,  but  it  may  be  pretty  safely  con- 
jectured to  be  fatty;  in  which  case,  the  action  of  the  liver,  so 
far  as  that  material  is  concerned,  would  be  the  preparation 
of  fat  from  nitrogenous  matters.  But  it  is  not  to  be  for- 
gotten that  an  excessive  secretion  of  bile  is  a  cause  of  purg- 
ing, and  may  thus  be  altogether  got  rid  of  by  the  bowel. 
The  functions  of  the  liver  may  therefore  be  grouped  under 
the  three  heads  of  arrest,  decomposition,  and  elimination. 

On  the  whole,  especially  when  the  large  size  of  the  liver 
before  birth  is  considered,  and  the  work  thrown  on  that  viscus 
in  the  case  of  Europeans  in  hot  climates,  it  may  be  hazarded 
as  a  conjecture  that,  besides  being  a  storehouse  for  non- 
nitrogenous  material  not  immediately  required,  the  liver  aids 
the  elimination  of  albuminoids  with  less  complete  decompo- 
sition, and  therefore  less  evolution  of  heat,  than  is  required 
to  decompose  them  in  the  tissues. 

127.  The  Kidneys. — The  kidneys  are  the  principal  elimi- 
nators of  nitrogenous  debris  and  of  salts  from  the  blood.  In 
the  human  subject  they  are  about  four  inches  long,  two  and 
a  half  broad,  and  flattened  from  before  backwards.  On  the 
inner  side  of  each  is  a  depression  called  the  hilus,  where  the 
renal  artery  enters,  and  the  vein  and  duct  emerge.  The 
duct  is  called  the  ureter  ;  it  is  dilated  at  its  commencement, 
but  diminishes  rapidly  to  the  size  of  a  goose  quill,  and  after 
a  course  of  fourteen  inches  or  more,  opens  into  the  lower 
part  of  the  urinary  bladder. 

The  kidney  is  a  tubular  gland,  and  in  all  classes  of  verte- 
brata  its  tubules  present,  at  or  near  their  origin,  small  grape- 
like  bodies,  called  Malpighiaii  corpuscles.  In  many  fishes 
the  two  kidneys  extend  the  whole  length  of  the  abdomen, 
from  the  head  backwards,  and  in  many  they  are  blended  in  one 
mass  behind.  In  the  other  vertebrata,  the  permanent  kidneys 
are  preceded,  in  the  embryo,  by  a  pair  of  primordial  kidneys 
or  Wolffian  bodies,  originally  extending  along  the  sides  of 
the  vertebral  column,  but  afterwards  confined  to  the  hinder 
part  of  the  abdominal  cavity.  These  consist,  like  the  per- 


ME    KIDNEYS. 


169 


manent  kidneys,  of  tubuli  uriniferi  commencing  in  dilata- 
tions; they  exercise  their  functions  for  a  limited  period,  and 
then  disappear  nearly  altogether;  but  their  ducts,  and  other 
tissue  connected  with  them,  are  closely  associated  with  the 
development  of  the  ducts  of  the  essential  reproductive  organs 
in  both  sexes.  In  reptiles  and  birds  the  kidneys  have  a  frilled 
or  convoluted  appearance;  in  mammals  they  are  more  com- 
pact than  in  any  other  vertebrata. 


Fig.  88.— WOLFFIAN  BODIES  OF 
EMBRYO  PIG.  a,  kidney;  b, 
Wolffian  body,  and,  on  its  sur- 
face, the  Miillerian  duct,  after-  Fig.  89. — KIDNEY  OF  KANGAROO, 
wards  the  Fallopian  tube  ;  c,  vertical  section.  a,  Ureter ; 
ovary  ;  d,  urinary  bladder  b,  cone,  to  the  summit  of 
turned  down  ;  e,  rectum.  which  all  the  tubules  are 
Double  size.  gathered. 

In  the  part  of  their  course  nearest  the  circumference  of 
the  organ,  the  tubules  of  the  mammalian  kidney  are  convo- 
luted, giving  a  granular  appearance  to  the  texture;  while  in 
the  inner  part  of  their  course  they  are  straight,  and  give  to 
the  texture  a  striated  appearance.  The  granular  outer  part 
is  called  the  cortical  portion  of  the  kidney,  and  the  striated 
inner  part  is  called  the  medullary  portion. 

128.  If  a  sheep's  or  a  rabbit's  kidney  be  examined,  the  cor- 
tical substance  will  be  seen  to  form  a  uniform  layer,  while  the 
straight  tubules  of  the  medullary  part  are  gathered  together 
in  one  group,  having  the  form  of  a  ridge  in  the  sheep,  and 
of  a  cone  in  the  rabbit  and  the  kangaroo,  projecting  into  the 
dilatation  from  which  the  ureter  springs. 

In  the  porpoise  each  kidney  consists  of  a  large  number  of 


170 


ANIMAL   PHYSIOLOGY. 


distinct  renules,  completely  separable  one  from  another,  and 
every  one  possessed  of  a  cortical  covering  and  a  medullary 
part  gathered  into  a  cone  like  that  of  a  rabbit's  kidney.  In 
the  seal  these  renules  are  distinct,  but  cannot  be  dissected 
separate;  while  in  the  ox  they  are  fused  more  closely  together, 
but  form  separate  lobules  on  the  surface. 

The  human  kidney  has  the  smooth  surface  of  that  of  the 
sheep,  but  in  construction  more  nearly  approaches  to  that  of 
the  ox.  Before  birth,  it  presents  a  lobulated  appearance 
of  the  surface  (fig.  85)  like  the  ox  kidney;  but  the  lobules 
become  so  fused  together  that  their  outlines  are  soon  oblite- 
rated, and  the  general  surface  made  smooth  like  the  kidney 
of  the  sheep.  The  construction,  however,  is  at  once  manifest 
on  laying  the  organ  open  by  a  vertical  section  from  the  hilus 
to  the  outer  border.  Then  it  is  seen  that  the  medullary 
substance  consists  of  a  number  of  cones  distinct  each  from 
the  rest,  being  separated  by  extensions  of  the  cortical  part. 


Fig.  90.— KIDNEY  OF  SEAL,  ver- 
tical section. 


Fig.  91. — HUMAN  KIDNEY,  ver- 
tical section." 


These  are  called  pyramids  of  Malpiglii  ;  and  while  their 
bases  are  imbedded  in  the  cortex,  the  apex  of  each  projects 
into  a  cup  or  calyx  which  embraces  it;  and  the  calyces 
open,  each  by  a  constricted  orifice  opposite  the  apex  of  the 
pyramid  which  it  embraces,  into  the  pelvis  of  the  kidney, 


THE   KIDNEYS. 


171 


a  common  cavity  in  the  hilus,  from  which  the  ureter  takes 
origin. 

129.  The  straight  tubes  of  which  each  pyramid  is  composed, 
open  by  a  group  of  orifices  at  the  summit,  and  divide  several 
times  as  they  pass  backwards.  They  are  the  continuations 
of  the  convoluted  tubes  which  form  the  cortex.  And  if  the 
texture  be  carefully  examined  in  the  deep  part  of  the  cortex, 
it  will  be  seen,  even  with  the  naked  eye,  particularly  in  a 
minutely  injected  kidney,  that  the  striated  appearance  pro- 
duced by  the  straight  tubes  of  the  pyramid  is  prolonged 
outwards  in  a  series  of  streaks,  each  of  which  is  imbedded  in 
the  granular-looking  convoluted  portions  of  the  tubes.  Such 
a  streak,  with  the  convoluted  tubes  belonging  to  it,  is  distin- 
guished as  a  pyramid  of  Ferrein.  Thus,  a  group  of  tubules 
forms  a  pyramid  of  Ferrein;  a  number  of  these  have  their 
straight  parts  prolonged,  and  unite  to  form  a  simple  kidney, 


Fig.  92. — TEXTURE  OF  KIDNEY,  semi- diagrammatic  view.  A,  Tubules 
and  Malpighian  corpuscles  of  two  pyramids  of  Ferrein ;  B,  afferent 
and  efferent  blood-vessels  of  Malpighian  corpuscles,  and  capillary 
networks. 


172  ANIMAL   PHYSIOLOGY. 

or  a  lobule  with  one  Malpighian  pyramid;  several  lobules 
may  become  blended  in  one  Malpighian  pyramid  and  larger 
lobule;  and,  lastly,  in  such  a  kidney  as  the  human,  the  lobules 
are  completely  fused  in  a  higher  unity. 

The  great  majority  of  the  uriniferous  tubules  are  about 
^.i_  inch  in  diameter;  near  the  summit  of  the  Malpighian 
pyramids,  however,  trunks  of  twice  or  thrice  that  size  are 
formed  by  the  union  of  others;  and  there  are  also  loops  of 
much  smaller  diameter,  each  of  which  is  intercalated  in  the 
course  of  a  convoluted  tubule,  and  descends  into  the  medul- 
lary substance. 

Where  the  change  of  arrangement  from  the  convoluted  to 
the  straight  condition  takes  place,  there  is  likewise  a  change 
of  structure;  for  the  columnar  or  rather  cubical  epithelial 
corpuscles  of  the  convoluted  tubes  are  turbid  masses  without 
distinct  outline,  while  those  of  the  straight  tubes  have  the 
appearance  of  cell  walls  round  them,  and  clear  contents;  a 

difference  which  suggests  that 
the  convoluted  tubes  are  more 
active  in  secretion,  or  take  at 
least  a  different  part. 

130.  Each  convoluted  tubule 
begins  in  a  little  spherical  body, 
named  after  the  same  old  ob- 
server as  the  pyramids,  Mal- 
pighian  corpuscles.  Each  of 
these  consists  of  a  capsule, 
which  is  the  dilated  commence- 
ment of  the  tubule,  and  a 
bunch  of  blood-vessels  called  a 
glomerulus ;  and,  when  the 
minute  blood-vessels  are  filled 
with  colouring  -  matter,  the 
Fig.  93.  — Relations  of  MAL-  glomeruli  are  distinguishable 
PIGHIAN  CORPUSCLE  to  tubule  with  the  n^ked  scattered 

and  blood-vessels.     1  odd  and     ,1.1  T      (1  ,•     -,        i 

Bowman.  ai*    through   the    cortical    sub- 

stance. Each  glomerulus  con- 
sists of  a  bunch  of  loops  of  blood-vessels  taking  origin  from  a 
single  little  afferent  artery  which  enters  the  capsule  at  the 
part  opposite  to  that  which  is  continued  into  the  tubule, 


THE  URINARY  BLADDER.  173 

and  pouring  their  contents  into  an  efferent  vessel  smaller 
than  the  afferent  one,  and  emerging  close  to  it.  These  efferent 
vessels  are  likewise  to  be  considered  as  arteries,  for,  on  their 
emergence,  they  break  up  into  capillaries,  which  supply  the 
whole  capillary  network  of  the  tubules  with  blood  which 
has  previously  passed  through  the  Malpighian  corpuscles. 
The  glomerulus,  when  distended,  completely  fills  the  capsule 
into  which  it  dips,  and  in  which  it  may  be  said  to  hang 
loose,  being  covered  with  only  a  delicate  layer  of  squamous 
epithelium. 

The  structure  of  the  Malpighian  corpuscles  leaves  little 
reasonable  doubt  as  to  their  function ;  they  are  fitted  to  allow 
water  to  drain  off  from  the  blood :  and  it  is  in  keeping  with 
this  hypothesis  that  they  are  very  rudimentary  in  birds,  as 
in  birds  the  urine  is  of  semi-solid  consistence,  forming  the 
white  part  of  their  mutings.  The  drain  of  water  from  the 
blood  in  the  glomerulus  accounts  for  the  efferent  vessel  being 
smaller  than  the  afferent ;  and  the  blood,  being  freed  from 
superfluous  water  first,  is  brought  in  a  more  concentrated 
state  into  contact  with  the  tubules,  whose  epithelia  remove 
from  it  the  solid  constituents  of  the  urine. 

131.  The  Urinary  Bladder  is  a  hollow  viscus,  with  its  outlet 
at  its  lower  part,  and  when  empty  it  lies  altogether  within  the 
pelvis,  but  when  distended  it  is  enlarged  in  an  upward  direc- 
tion, rising  considerably  above  the  pelvis,  and  resting  against 
the  abdominal  wall,  to  which  it  is  bound  down  by  peritoneum. 
In  that  state  it  is  comparatively  unprotected;  and  when  over- 
distended,  it  has  been  known  to  be  ruptured  by  very  slight 
accidental  violence.  The  ureters  open  into  the  bladder 
behind  its  orifice  or  neck,  their  points  of  entrance  forming 
with  that  orifice  the  angles  of  an  equilateral  triangle,  with 
sides  about  1J  inches  long,  and  called  the  trigone.  The 
mucous  membrane  of  the  bladder  has  a  stratified  squamous 
epithelium,  while  that  of  the  ureters  is  intermediate  between 
the  squamous  and  columnar  varieties.  At  the  orifices  of  the 
ureters,  which  enter  the  bladder  obliquely,  the  mucous  mem- 
brane is  somewhat  redundant,  so  as  to  form  on  each  orifice 
a  valve  which  effectually  prevents  regurgitation. 

The  muscular  walls  of  the  bladder  have  the  fibres  extend- 
ing round  it  in  every  variety  of  direction,  yet  not  without 


174  ANIMAL   PHYSIOLOGY. 

definite  arrangement;  for  they  can  be  shown  to  consist  of 
multitudes  of  figures  of  eight,  with  the  crossings  of  the  figures 
in  front  and  behind,  and  their  loops  round  the  summit  and 
outlet  (Pettigrew).  The  fibres  are  of  the  unstriped  descrip- 
tion ;  but  round  the  urethra  or  canal  of  exit,  where  it  emerges 
from  the  pelvis,  there  are  muscles  of  the  striped  kind,  by 
whose  action  the  contents  of  the  bladder  are  voluntarily 
retained,  when  its  walls  are  irritated  to  contract  by  the 
stimulus  of  distension. 

132.  The  Urine. — The  average  amount  secreted  by  the 
kidneys  is  about  fifty  fluid  ounces,  or  two  pints  and  a  half, 
in  the  day;  the  reaction  is  usually  acid,  and  the  average 
specific  gravity  is  1*020;  but  the  amount  of  water  secreted 
by  this  channel  is  affected  not  only  by  the  amount  imbibed, 
but  by  various  circumstances,  particularly  temperature  and 
exercise,  which  influence  the  quantity  carried  away  by  the 
skin  and  lungs.  Whatever  diminishes  the  amount  of  water 
secreted  by  the  kidneys,  or  increases  the  quantity  of  waste 
substance,  heightens  the  specific  gravity  of  the  urine. 

The  solid  contents  of  the  urine  consist  of  nitrogenous 
matters  and  salts.  The  salts  are  chlorides,  sulphates,  and 
phosphates.  Chlorides  occur  in  all  the  fluids  of  the  body; 
the  sulphates  take  origin,  no  doubt,  by  the  oxidation  of  the 
sulphur  contained  in  albuminoid  matters;  and  the  phosphates 
are  derived  from  the  oxidation  of  the  phosphorus  in  albumi- 
noids, and  in  the  protagon  of  the  nervous  system,  and  from 
the  phosphates  pre-existing  in  the  bones  and  various  fluids. 

The  principal  nitrogenous  matter  in  the  urine  is  urea, 
which,  as  already  stated,  is  the  substance  in  the  form  of 
which  by  far  the  greater  part  of  the  nitrogen  escapes  from 
the  body.  It  is  exceedingly  soluble,  and  contains  exactly 
one  half  its  weight  of  nitrogen.  About  500  grains  of  urea 
may  be  said  to  escape  by  the  kidneys  in  twenty-four  hours. 
There  is  contained  in  the  urine  of  man  a  small  quantity  oi 
hippuric  acid,  a  substance  which  gets  its  name  from  being 
found  abundantly  in  the  urine  of  the  horse;  and  there  is 
likewise  ordinarily  a  small  quantity  of  uric  acid,  which  is 
important;  because,  from  slight  derangements  of  nutrition, 
its  amount  is  liable  to  increase,  and  when  it  accumulates 
in  the  blood  it  produces  gout,  while,  in  the  urine,  on 


THE    URINE.  175 

account  of  its  sparing  solubility,  it  may  lead  to  stone  in  the 
bladder. 

It  may  be  further  remarked,  that  in  animals  which  void 
the  urine  solid,  uric  acid  takes  the  place  of  urea;  thus  from 
serpents  uric  acid  is  obtained  in  masses  of  pure  crystals.  Other 
matters  are  contained  in  the  urine  in  small  quantity,  such  as 
a  peculiar  pigment,  various  extractive  matters,  among  which 
may  be  mentioned  kreatin,  and  mucus  from  the  bladder. 

The  kidney  is  both  a  separator  of  urea  from  the  blood,  and 
a  manufacturer  of  more.  That  some  of  the  urea  is  formed 
elsewhere,  and  carried  to  the  kidneys  in  the  circulating 
stream,  is  shown  from  that  substance  being  always  present  in 
the  blood,  although  in  small  quantity,  and  from  its  accumu- 
lating in  animals  from  which  the  kidneys  have  been  extracted. 
But  that  it  is  in  large  part  manufactured  by  the  kidneys,  is 
proved  by  its  accumulating  far  more  rapidly  in  animals 
whose  ureters  have  been  tied,  than  in  those  that  have  had 
the  kidneys  removed. 


CHAPTER  XIII. 

THE  NERVOUS  SYSTEM. 

133.  In  the  working  of  a  nervous  system  in  any  animal, 
there  are  three  sets  of  parts  involved;  namely,  the  nervous 
centre,  the  terminal  organ,  and  the  link  of  communication 
between  the  two,  namely,  the  nerve.  The  distinctive  part  of 
every  nervous  centre  consists  of  nucleated  corpuscles,  and  any 
nervous  mass  containing  nerve-corpuscles  is  called  a  ganglion. 
The  nerve  consists  of  uninterrupted  fibres  in  structural  con- 
tinuity with  the  corpuscles,  and  without  any  branching  until 
close  to  their  termination;  and  the  terminal  organs  are  like- 
wise in  structural  continuity  with  the  nerves. 

These  terminal  organs  are,  however,  of  very  various 
descriptions,  and  with  as  much  claim,  in  many  instances,  to 
be  separately  grouped  as  to  be  considered  along  with  the 
nervous  system  to  which  they  are  so  intimately  united.  For 
example,  it  would  be  difficult  to  raise  a  valid  objection  to  con- 
sidering muscular  fibres  in  their  entirety  as  terminal  organs 
of  nerves;  yet  they  have  a  development  and  function  of  their 
own,  and  it  would  be  inconvenient,  as  well  as  erroneous,  to 
look  on  them  as  mere  parts  of  the  nervous  system  which 
governs  them.  It  will  be  recollected  that  in  treating  of  the 
skin,  several  terminal  nervous  organs  have  already  been 
described  (p.  68),  to  which  the  integuments  owe  their  sensi- 
bility; and  more  complex  organs  are  devoted  to  the  special 
senses,  which  will  hereafter  be  described.  But  at  present 
we  shall  consider  only  the  nervous  centres  and  the  nerves. 

The  nervous  system,  as  developed  in  the  invertebrate 
animals,  consists  of  a  series  of  ganglia  connected  in  chains 
or  other  groups,  and  giving  off  nerves;  but  in  the  vertebrata 
it  is  divisible  into  two  parts.  One  of  these  is  the  cerebro- 
spinal  system,  consisting  of  the  brain  and  spinal  cord;  together 


THE   NERVOUS    SYSTEM.  177 

termed  the  cerebro  spinal  axis,  and  of  the  nerves  issuing 
therefrom,  which  supply  the  voluntary  muscles,  the  integu- 
ments and  organs  of  special  sense,  and  various  other  struc- 
tures; the  other  is  called  the  sympathetic  system,  and  consists 


Fig.  94.—  CEREBRO-SPI^AI,  NERVOUS  SYSTEM. 
H 


178    .  ANIMAL   PHYSIOLOGY. 

of  chains  and  irregular  collections  of  ganglia,  and  of  nerves 
supplied  principally  to  viscera  and  blood-vessels.* 

The  rule  that  the  nerve-fibres  remain  distinct,  each  from 
every  other,  and  never  branch  until  within  a  microscopic 
distance  of  their  termination,  holds  good  of  both  cerebro- 
spinal  and  sympathetic  systems;  but  nerves  consist  of 
bundles  of  fibres,  which  may  be  re-arranged  in  other  bundles, 
forming  what  is  called  a  plexus;  and  while  in  the  cerebro- 
spinal  system  these  plexuses  are  comparatively  simple,  and 
confined  to  the  large  trunks  near  their  origin  and  a  few  fine 
twigs  here  and  there,  in  the  sympathetic  system  they  are 
abundant  and  complex,  giving  a  meshed  and  intricate  appear- 
ance to  the  bundles,  which  are  seldom  collected  in  mode- 
rately large  trunks. 

134.  Nervous  Action. — The  simplest  idea  of  the  use  of 
the  nervous  system  is  got  from  what  is  termed  reflex  action, 
because  that  is  uncomplicated  with  consciousness.  In  a 
reflex  action  an  irritation  is  applied  to  a  part,  and  produces 
in  a  nerve  a  change  of  condition,  which  is  called  an  impres- 
sion; this  impression  is  termed  sensory  or  centripetal,  and 
travels  to  the  nerve-centre  with  which  the  nerve  is  connected, 
and  thence  it  is  reflected  along  some  other  nerve-fibre,  and 
takes  a  centrifugal  course  to  a  muscle  or  other  organ,  which 
it  stimulates  to  action.  If  the  organ  so  stimulated  be  a 
muscle,  the  nervous  action  is  excito-motor ;  if  it  be  a  secreting 
cell,  the  action  is  excito-secretory ;  if  it  be  an  electric  organ, 
such  as  exists  in  various  fishes,  the  nervous  action  excites  it 
to  give  a  shock  of  electricity.  But  in  every  case  it  is  one 
description  of  change  which,  under  the  name  of  an  impression, 
passes  up  one  nerve,  through  the  nerve-centre,  and  down 

*  The  typical  arrangement  of  nervous  system  found  in  segmented 
invertebrata  is  a  chain  of  ganglia  running  along  the  lower  part  of  the 
body,  with  the  foremost  placed  in  front  or  above  the  opening  of  the 
mouth;  and  the  question  naturally  arises,  what  relation  has  this 
chain  to  the  vertebrate  nervous  system?  Now,  certain  researches 
seem  to  point  out  a  relationship  between  vertebrate  and  invertebrate 
animals  through  the  tunicate  molluscs.  These  molluscs  have  only 
one  ganglion,  which,  I  believe,  may  be  fairly  considered  as  homo- 
logous with  the  anterior  or  pre-cesophageal  ganglion  of  articulataj 
and  it  appears  most  probable  that  the  cerebro- spinal  axis  of  verte- 
brata  is  a  highly  developed  structure  corresponding  with  that  one 
ganglion, 


NERVOUS   ACTION.  179 

another  nerve  to  reach  the  terminal  organ  ;  while  the  effect 
produced  depends  on  the  intrinsic  properties  of  that  organ. 
It  is  also  interesting  to  notice  that  in  such  an  action  there  is 
no  generation  of  new  force;  but  the  irritant,  or  stimulus, 
gives  rise  to  or  is  converted  into  a  nervous  impression,  and 
the  impression,  after  travelling  through  a  circuit,  is  followed 
by  the  action  of  the  terminal  organ. 

It  is  by  such  reflex  action  that  a  flow  of  saliva  follows  the 
introduction  of  sapid  substances  into  the  mouth.  The  sapid 
substance  irritates  the  nerves  of  the  mouth,  and  the  impres- 
sions travel  along  them  to  a  nervous  centre,  whence  they  are 
reflected  along  the  nerves  terminating  in  the  secreting  cor- 
puscles of  the  gland.  So,  also,  the  muscles  of  the  stomach  and 
intestines  are  excited  to  regular  movements,  not  by  the  direct 
influence  of  the  food,  but  by  impressions  originated  in  nerve 
extremities,  conducted  to  centres,  and  thence  reflected. 
Direct  irritation  of'  muscle  produces  mere  spasmodic  con- 
traction of  the  muscular  fibres  irritated;  but  the  impressions 
sent  from  nervous  centres  produce  a  co-ordinated  action  of 
numbers  of  fibres,  suited  to  the  accomplishment  of  some 
definite  end.  Thair,  no  doubt,  is  a  result  not  thoroughly 
understood;  but  it  is  connected  with  the  circumstance  that  a 
nerve-centre  always  consists  of  numbers  of  corpuscles,  and 
gives  off  a  number  of  fibres. 

Impressions  sent  to  nervous  centres  are  not,  however, 
always  reflected  to  the  region  whence  they  came.  They  may 
pass  to  other  nervous  centres,  and  lead  to  changes  in  all  parts 
of  the  body;  and,  when  they  reach  the  brain,  they  produce 
sensation. 

Here  we  sere  brought  into  contact  with  an  apparent  compli- 
cation in  the  functions  of  the  nervous  system,  its  relations  to 
consciousness.  These  relations  are  exhibited  in  two  ways, 
one  is  the  production  of  sensation  by  nervous  action  reaching 
the  brain;  the  other  is  the  production  of  nervous  action 
initiated  in  the  brain  by  volition.  It  is,  however,  to  be 
clearly  understood  that  the  brain  is  only  a  complex 
arrangement  of  nerves  and  nerve-corpuscles,  comparable  with 
the  simpler  nervous  centres;  and  although  it  both  affects  and 
is  affected  by  the  mind,  there  is  no  reason  to  believe  that  its 
active  condition  is  dissimilar  from  that  of  nerves  and  nerve- 


180  ANIMAL   PHYSIOLOGY. 

corpuscles  throughout  the  rest  of  the  nervous  system.  Its 
peculiarity  lies  in  a  sympathy  between  it  and  the  niind, 
whereby  centripetal  action  which  has  reached  it  affects  the 
mind,  and  the  mind  in  turn  excites  nervous  action  in  it, 
which  may  extend  centrifugally  to  the  voluntary  muscles. 

The  sum  of  the  function  of  the  nervous  system  is,  that  it 
conveys  and  distributes  nervous  impression.  The  waves  of 
impressed  condition,  or  nervous  impulses,  may  originate  either 
from  external  irritation  applied  to  terminal  organs,  or  from 
an  unknown  description  of  irritation  exerted  by  the  mind ; 
and  in  some  instances  they  influence  the  mind,  while  in 
others  they  act  on  muscles  and  glands.  Further,  it  must  be 
admitted  that  no  mental  action  can  take  place  without 
nervous  action;  but  it  is  distinctly  to  be  understood  that 
nervous  impression,  or  the  active  state  of  the  nervous  system, 
is  not  the  same  thing  as  mental  action,  but  is  a  definite 
physical  change,  which,  there  is  no  reason  to  doubt,  is  always 
of  one  description. 

135.  Living  nerve  resembles  living  muscle  in  being  in  a 
peculiar  state  of  electric  tension  when  at  rest,  and  in  chang- 
ing its  electric  condition  when  in  action.  Detached  portions 
of  nerve  examined  by  means  of  the  galvanometer  give  results 
similar  to  those  obtained  from  blocks  of  muscle  (p.  57);  but 
the  currents  being  much  weaker,  a  finer  instrument  is  required 
for  their  detection.  Nerve  has  further  the  remarkable  pro- 
perty, that  if  a  portion  be  placed  within  the  circuit  of  a 
galvanic  current,  a  current  in  the  same  direction  is  produced 
in  the  whole  length  of  the  nerve :  the  nerve  is  then  said  to 
be  in  a  state  of  electrotonus.  A  galvanic  current  applied  to 
a  trunk  of  nerve  supplying  a  muscle  does  not  maintain  the 
muscle  in  contraction ;  but  there  is  a  contraction  every  time 
that  the  circuit  is  completed,  and  every  time  that  it  is  broken, 
so  that  the  muscle  can  only  be  kept  contracf ed  by  a  con- 
stantly interrupted  current.  In  electrotonus  the  nerve  still 
performs  its  functions,  but  its  degree  of  irritability  is  altered 
in  different  parts  of  its  course.  These  facts  show  that  nervous 
influence  is  not  a  current  of  electricity.  By  applying  the 
stimulus  to  different  points  on  the  nerve  trunk,  and  measur- 
ing, with  an  instrument  called  a  myographion,  the  difference 
in  time  between  application  of  the  stimulus  and  contraction 


NERVOUS   TISSUES. 


181 


of  the  muscle,  the  rate  can  be  demonstrated  at  which  the 
nervous  influence  or  impression  travels.  The  rate  is  estimated 
at  more  than  100  feet  per  second  in  the  human  subject;  but 
it  is  known  to  vary  at  different  times  in  the  same  nerve. 

136.  Nervous  Tissues. — Nerve-fibres  present  considerable 
variety  of  microscopic  appearance.  But  those  which  are 
largest,  and  found  most  abundantly  in  nerve  trunks  taking 
origin  from  the  brain  and  spinal  cord,  are  the  medullated 
fibres.  These  may  have  a  diameter  as  great  as  YTTTTF  °^  an 
inch.  They  present  a  limiting  membrane,  with  nuclei 

3 


Fig.  95. — NERVE-FIBRES.     «,  Medullated  fibre  in  the  fresh  state ; 

b,  similar  fibre  showing  the  nuclei  of  the  limiting  membrane ; 

c,  axis -cylinder  projecting  beyond  the  torn  limiting  membrane  of 
a  fibre,   from  which  the  medullary  sheath  has  been  partially 
removed ;  d,  substance  of  medullary  sheath  escaping  in  irregular 
drops  ;  e,  fibre  treated  with  acetic  acid  ;  /,  fine  medullated  fibre 
from  brain,  the  medullary  sheath  running  into  drops,  being  un- 
supported by  any  limiting  membrane  ;  g,  grey  fibres. 

scattered  on  it,  like  the  sarcolemma  of  muscle,  and  inside  of 
this,  in  the  fresh  state,  a  clear  and  seemingly  homogeneous 
substance,  which  oozes  out  in  a  semifluid  fashion  from  places 
where  the  limiting  membrane  is  ruptured.  But  after  a  short 
time,  or  on  addition  of  reagents,  such  as  a  drop  of  acetic  acid, 
a  coagulation  takes  place,  and  there  are  then  exhibited 
a  thin  central  thread,  the  axis-cylinder,  in  the  middle,  and 
around  this  a  substance  which  is  called  from  its  opacity  when 


182  ANIMAL   PHYSIOLOGY. 

coagulated,  white  substance  of  Schwann,  and  from  its  marrow- 
like  consistency,  the  medullary  sheath.  The  limiting  mem- 
brane is  absent  from  the  fibres  within  the  brain  and  spinal 
cord.  The  axis-cylinder  is  of  albuminoid  composition,  while 
the  white  substance  is  rich  in  protagon,  a  material  containing 
phosphorus,  soluble  in  warm  alcohol,  but  not  in  ether,  and 
readily  yielding  fatty  compounds  by  decomposition.  Within 
its  nervous  centre  a  medullated  fibre  often  becomes  much 
reduced  in  diameter,  and  at  its  peripheral  termination  it  may 
consist  of  axis-cylinder  without  any  white  substance,  and 
may  break  up  into  branches.  The  axis-cylinder,  when  care- 
fully examined,  exhibits  a  longitudinal  striation,  considered 
by  some  observers  as  indicating  a  bundle  of  primitive  fibrils 
which,  by  separating,  give  the  appearance  of  branching 
referred  to.  It  is,  however,  to  be  remarked  that  by  the 
nitrate  of  silver  method,  a  transverse  striation  may  be  like- 
wise exhibited;  and  as  in  the  case  of  muscular  fibre,  so  also 
in  that  of  axis-cylinders,  it  is  doubtful  what  importance  is  to 
be  attached  to  either  longitudinal  or  transverse  markings. 

Another  set  of  iion-medullated  fibres  of  soft  consistence, 
darker  than  the  medullated  fibres,  distinguished  as  grey  fibres, 
and  presenting  large  nuclei,  which  occupy  their  whole 
breadth,  are  found  mingled  with  medullated  fibres  most 
abundantly  in  the  sympathetic  system,  and  also  constitute 
all  the  filaments  of  the  nerve  of  smell.  A  doubt  may  be 
entertained  as  to  whether  the  nuclei  are  imbedded  in  their 
protoplasmic  substance,  or  belong  to  a  sheath.  But  what  is 
most  important  for  the  student  to  understand  is,  that  every 
nerve  in  the  body  has  an  albuminoid  thread,  while  only  some 
are  enveloped  in  the  substance  yielding  protagon. 

137.  Nerve-corpuscles,  improperly  called  nerve -cells,  are 
sometimes  surrounded  with  a  nucleated  sheath,  but  they 
have  no  cell  wall.  They  consist  of  a  clear  nucleus,  and  one 
or  more  nucleoli  imbedded  in  a  mass  of  protoplasm  loaded 
with  granules.  They  send  out,  in  probably  every  instance, 
processes  termed  poles.  Some  of  these  poles  are  continuous 
with  nerves,  others  communicate  with  other  corpuscles;  while, 
in  the  case  of  multipolar  corpuscles,  many  of  them  would 
appear  to  ramify  till  the  branches  reach  an  extreme  tenuity. 

The  firmness  of  the  different  parts  of  the  nervous  system 


NERVOUS   TISSUES. 


183 


depends  much  more  upon  the  connective  tissue  present,  than 
on  the  nervous  elements.  The  nerves  in  their  course  through 
the  tissues  are  tough  strong  cords,  which  bear  a  strong  pull 
in  dissection,  because  the  fibres  are  bound  together  by  means 
of  firm  sheaths  of  connective  tissue;  and  for  a  similar  reason 
the  ganglia  of  the  sympathetic  system  are  exceedingly  tough. 


Fig.  9G. — MULTIPOLAR   NERVE  -  CORPUSCLE  from  anterior  cornu  of 
spinal  cord  of  ox.     Schultze  after  Deiters. 

But  the  brain  and  spinal  cord  are  of  a  soft  consistence,  easily 
destroyed  by  handling,  and  reduced  to  viscid  pulp  by  a  little 


184  ANIMAL   PHYSIOLOGY. 

pressure,  because  the  substance  which  binds  their  nervous 
elements  is  nearly  as  delicate  as  those  elements  themselves. 
The  nerves  emerging  from  the  brain  and  spinal  cord  are 
likewise  delicate  before  piercing  the  fibrous  membrane  (dura 
mater)  which  surrounds  those  structures  and  mingles  its  fibres 
with  the  nerve  trunks  as  they  issue  from  it. 

138.  Cerebro-Spinal  Axis. — The  brain  and  spinal  cord 
form  one  continuous  structure,  the  cerebro-spinal  axis;  and 
this,  in  early  embryonic  life,  as  it  may  be  studied,  for 
example,  in  a  hen's  egg  which  has  been  hatched  for  two  or 
three  days,  lies  originally  in  an  open  groove  on  the  sur- 
face of  the  body  (fig.  149).  But  soon  the  edges  of  the 
groove  come  together,  converting  it  into  a  tube,  which  is  the 
future  cranial  cavity  and  spinal  canal;  and  in  like  manner 
the  edges  of  the  structure  within  the  canal  unite,  and  the 
cerebro-spinal  axis  thus  becomes  tubular  also  (fig.  153).  But 
the  part  which  forms  the  brain  is  very  unequally  developed, 
being  bent  on  itself,  its  cavity  swollen  at  some  parts,  con- 
stricted at  others,  and  its  walls  thin  or  absent  at  one  place 
and  thick  at  another;  while  the  spinal  cord  retains  its  cylin- 
drical shape,  although  its  walls  are  thickened,  and  its  hollow, 
which  remains  as  the  central  canal,  is  so  small  that  it  cannot 
be  seen  in  the  human  subject  without  the  aid  of  a  micro- 
scope. 

Two  descriptions  of  substance  are  distinguished  in  the 
cerebro-spinal  axis,  the  white  and  the  grey  or  cineritious. 
The  white  matter  consists  of  nerves  without  corpuscles,  and 
owes  its  whiteness  to  the  medullary  sheaths  of  the  nerves; 
while  the  grey  matter  has  only  very  minute  medullated 
fibres,  and  contains  in  different  parts  a  great  variety  of 
dispositions  of  nerve  -  corpuscles.  Notwithstanding  these 
varieties  of  the  grey  matter,  and  that  the  difference  in 
colour  between  it  and  the  white  depends  on  the  greater 
vascularity  of  the  grey,  and  on  the  smaller  proportion  borne 
by  the  medullary  sheaths  to  the  other  textural  elements; 
yet,  as  tracts  of  grey  matter  without  corpuscles  are  very 
small,  and  the  white  matter  has  no  corpuscles  anywhere,  the 
grey  matter  may  be  considered  as  ganglionic,  and  the  white 
as  merely  conducting.  Along  the  whole  length  of  the 
original  cerebro-spinal  cylinder,  grey  matter  lies  next  the 


CEREBRO-SPINAL.  AXIS.  185 

interior  of  the  tube,  while  the  white  matter  is  exterior;  but 
in  the  brain  there  are  extensive  additional  developments  of 
grey  matter  on  the  convoluted  surface  of  the  cerebrum  and 
cerebellum. 

139.  Large  blood-vessels  appear  to  be  inadmissible  in  the 
substance  of  the  cerebro-spinal  axis,  for,  instead  of  having 
vessels  ramifying  through  it  as  in  other  textures,  it  is  closely 
invested  with  a  vascular  membrane  called  pia  mater,  which 
consists  of  thickly  meshed  branches  of  arteries  and  veins 
united  with  a  little  connective  tissue;  and  from  this  mem- 
brane, which  dips  down  into  every  fissure,  multitudes  of 
small  arteries  enter  all  over  the  surface,  so  minute  that,  in  a 
section  of  the  brain,  the  only  vessels  visible  to  the  naked 
eye  appear  as  scattered  spots  of  blood  no  larger  than  marks 
made  with  the  point  of  a  pin. 

Over  the  pia  mater  there  is  a  delicate  transparent  serous 
membrane,  called  the  arachnoid,  with  its  serous  surface 
turned  outwards,  and  stretched  across  the  various  inequalities 
of  surface,  without  dipping  into  them.  It  is  adherent  to  the 
pia  mater  over  great  part  of  the  surface  of  the  brain,  but  on 
the  spinal  cord  is  disposed  as  a  loose  bag. 

Superficial  to  the  arachnoid  is  placed  the  dura  mater, 
an-  exceedingly  tough  fibrous  membrane,  which,  within  the 
cranium,  serves  as  periosteum  to  the  interior  of  the  skull,  as 
well  as  for  an  envelope  to  the  brain;  but  in  the  spinal  canal 
is  separated  from  the  vertebrae  by  a  space  containing  loose 
adipose  tissue  and  large  veins.  Its  outer  surface  is  rough, 
but  the  inner  is  polished  and  clothed  with  epithelium,  com- 
pleting with  the  opposed  arachnoid  membrane  the  boundaries 
of  a  serous  cavity,  called  the  arachnoid  space,  in  contradis- 
tinction to  the  subarachnoid  space  between  the  arachnoid  and 
pia  mater. 

The  spinal  arachnoid  is  attached  to  the  dura  mater  on 
each  side,  not  only  by  the  sheaths  with  which  it  clothes  the 
nerves,  but  by  a  series  of  attachments,  one  between  each 
pair  of  successive  nerves,  constituting  the  ligamentum  denti- 
culatum.  When  it  is  added  that  the  subarachnoid  space 
contains  a  considerable  amount  of  watery  secretion,  the 
cerebro-spinal  fluid,  principally  in  the  spinal  canal,  the 
student  will  perceive  that  the  spinal  cord  has  in  this,  and 


186 


ANIMAL   PHYSIOLOGY. 


,  in  its  loose  coverings,  in  the 
™  middle  of  which,  it  is  re- 
tained by  the  ligamenta 
ad  denticulata  and  the  passage 
outwards  of  its  nerves,  an 
efficient  protection  which 
accounts  for  its  immunity 
from  damage  in  movements 
of  the  vertebral  column.  The 
brain,  on  the  other  hand,  fits 
exactly  within  the  cavity  of 
the  unyielding  cranium. 

140.  The  Spinal  Cord  ex- 
tends from  the  skull  to  the 
level  of  the  first  lumbar  ver- 
tebra, and  is  from  fifteen  to 
eighteen  inches  long.  It  is 
about  the  thickness  of  the 
little  finger,  but  is  broader 
from  side  to  side  at  the  part 
of  the  neck  where  the  great 
nerves  pass  off  to  supply  the 
arms,  and  presents  another 
thickening  at  its  lower  end, 

Fig.  97.  —  SPINAL  CORD.  A, 
Transverse  section  in  cervical 
region ;  a  r,  anterior  root  of 
epinal  nerve  ;  p  r,  posterior 
root ;  g,  spinal  ganglion  ;  a  d, 
anterior  division  of  nerve;  pd, 
posterior  division.  B,  Front 
view  of  a  portion  in  the  dorsal 
region,  with  the  dura  mater 
laid  open,  and  the  anterior 
roots  of  nerve  on  the  right 
side  divided;  I  I,  ligamentum 
denticulatum.  C,  Front  view 
of  the  extremity  of  the  cord 
and  part  of  the  cauda  equina. 
The  roots  of  the  nerves  are 
divided  on  the  left  side;  ft, 
filum  terminale.  The  arachnoid 
is  omitted  in  these  figures, 


SPINAL   COED. 


is? 


•where  the  trunks  to  the  lower  limbs  take  origin.  It  is 
divided  into  two  symmetrical  parts  by  a  deep  anterior 
median  fissure,  and  by  what  is  called  the  posterior  median 
fissure,  which  is  deeper  than  the  anterior,  but  is  not  a 
true  fissure,  being  only  a  septum  of  the  proper  connective 
tissue  of  the  cord  with  larger  vessels  in  it  than  are  found 
in  the  nervous  substance.  Between  these  two  fissures 
is  the  microscopically  small  central  canal,  situated  in  grey 
matter  constituting  what  is  called  the  grey  commissure, 
in  contradistinction  to  a  little  white  matter  in  front  of  it, 
called  the  white  commissure.  On  each  side,  the  grey  com- 
missure spreads  out  into  an  extended  mass  of  grey  matter, 


Fig.  98. — SPINAL  CORD,  transverse  section;  a,  anterior  fissure;  6, 
central  canal ;  c,  grey  commissure ;  d,  white  commissure ;  e,  e, 
bundles  of  anterior  root  of  nerve  coming  from  the  anterior  cornu 
of  the  grey  matter ;  /,  posterior  root  passing  in  to  the  posterior 


188  ANIMAL   PHYSIOLOGY. 

which,  in  transverse  section,  is  seen  to  project  forwards  and 
backwards,  constituting  the  anterior  and  posterior  cornua. 

The  spinal  nerves  are  attached  to  the  cord  symmetrically, 
and  emerge  from  the  spinal  canal  by  the  intervertebral  fora- 
mina, one  on  each  side,  between  each  pair  of  successive 
vertebra?.  There  are  thirty-one  pairs  of  them,  the  upper- 
most emerging  between  the  atlas  and  skull,  and  the  lowermost 
behind  the  coccyx;  and  while  the  upper  pairs  pass  directly 
outwards  to  pierce  the  dura  mater,  the  lower  pairs,  including 
the  large  nerves  for  the  lower  limbs,  come  off,  crowded 
together,  from  the  lower  end  of  the  cord,  and  descend  to 
their  apertures  of  exit  in  a  bundle,  called  cauda  equina,  in- 
vested with  a  common  bag  of  arachnoid.  The  first  eight 
spinal  nerves  are  termed  cervical,  then  follow  twelve  dorsal, 
five  lumbar,  and  five  sacral,  each  one  of  which  is  named  after 
the  vertebra  above  it;  and  lastly,  there  is  a  small  coccygeal 
pair. 

Each  spinal  nerve  arises  by  an  anterior  and  a  posterior 
root,  which  emerge  from  the  cord  in  two  vertical  series  of 
fibres;  and  when  the  cord  is  cut  across,  the  fibres  of  the 
posterior  roots,  which  enter  at  the  level  of  the  section,  are 
seen  to  pass  in  to  the  posterior  cornu  so  compactly  as  to 
divide  a  posterior  column  of  the  white  matter  of  the  cord 
completely  from  the  rest;  while  the  fibres  of  the  anterior 
root  pass  to  the  anterior  cornu  in  scattered  bundles,  which 
make  by  no  means  so  definite  a  separation  between  what  are 
termed  the  anterior  and  lateral  columns. 

The  posterior  roots  are  larger  than  the  anterior,  and  have 
each  a  ganglion  situated  in  the  invertebral  foramen,  imme- 
diately internal  to  the  point  where  the  anterior  and  posterior 
roots  unite;  but  the  use  of  these  spinal  ganglia  is  unknown. 
No  sooner  are  the  fibres  of  the  roots  united  in  one  trunk 
than  they  divide  into  an  anterior  and  posterior  division, 
each  containing  fibres  from  both  roots.  The  posterior  divi- 
sions supply  the  muscles  which  erect  the  back  and  head,  and 
also  a  tract  of  integument  extending  from  the  crown  of  the 
head  the  whole  length  of  the  back.  The  anterior  divisions 
supply  the  whole  of  the  rest  of  the  body:  those  between  the 
ribs  are  termed  intercostal,  and  pass  separately  round  the 
visceral  cavity,  between  the  intercostal  muscles,  giving  off 


THE    SPINAL   CORD.  189 

cutaneous  branches  in  front  and  at  the  sides;  but  most  of 
the  rest  are  united  in  plexuses — the  first  four  cervical  form- 
ing the  cervical  plexus;  the  last  four  cervical,  and  greater 
part  of  the  first  dorsal,  joining  to  make  the  brachial  plexus 
for  the  supply  of  the  upper  limb;  and  the  lumbar  and  four 
upper  sacral  nerves  making  the  lumbar  and  sacral  plexuses, 
principally  distributed  to  the  lower  limb. 

141.  The  functions  of  the  anterior  and  posterior  roots  can 
be  exhibited  by  vivisection  only.     If  the  spinal  canal  of  an 
animal  be  laid  open,  and  the  posterior  roots  of  the  nerves 
going  to  one  of  its  limbs  be  divided,  while  the  anterior  roots 
are  left  uninjured,  the  animal  will  continue  to  move  the 
limb  and  walk  on  it  as  if  nothing  had  happened ;  but  the 
limb  may  be  pinched,  burned,  or  cut,  without  any  sign  of 
suffering  being  produced.      The  distal  end  of  the  divided 
posterior  roots  may  be  irritated  freely  without  any  effect  of 
any  sort  being  apparent;  but  if  irritation  be  applied  to  the 
end  in  connection  with  the  cord,  the  animal  will  give  unmis- 
takable signs  of  acute  pain.     If,   however,  instead  of  the 
posterior  roots,  the  anterior  roots  be  divided,  the  limb  which 
the  nerves  supply  will  become  immediately  powerless;  the 
animal  will  be  no  longer  able  to  move  it,  and  it  will  hang 
flaccid;  but,  though  paralysed  in  respect  of  motion,  it  will 
exhibit  no  paralysis  of  sensation,  for  the  animal  will  show 
as  much  sign  of  pain  when  that  limb  is  pinched  as  if  the 
nerves  had  not  been  touched.    When  the  ends  of  the  divided 
anterior  roots  in  connection  with  the  cord  are  irritated,  no 
effect  is  produced;  but  when  the  other  ends  are  irritated,  the 
muscles  of  the  limb  are  contracted  and  spasmodic  move- 
ments take  place.     From  such  experiments  as  these,  it  is 
concluded  that  the  anterior  roots  of  the  spinal  nerves  are 
motor,  and  the  posterior  roots" sensory. 

142.  Experiment,  which  reveals  thus  much  with  regard  to 
the  spinal  nerves,  throws  light  also  on  the  functions  of  the 
spinal  cord.     If  the  cord  be  cut  right  across,  the  animal 
ceases   to  have  any  feeling  in  the   regions   supplied  with 
nerves  given  off  from  the  part  of  the  cord  severed  from  com- 
munication with  the  brain,  and  has  no  longer  any  power  to 
move  them;    the  cord  is,  therefore,   the  sole  conductor  of 
impressions  to  and  from  the  spinal  nerves.     But  it  is  also  a 


190  ANIMAL  PHYSIOLOGY. 

nervous  centre  taking  part  in  reflex  actions;  for,  if  tlie 
paralysed  limbs  be  pinched  or  otherwise  irritated,  their 
muscles  are  contracted  so  as  to  draw  them  up,  as  if  under 
the  influence  of  the  will,  but  without  the  animal  evincing 
any  knowledge  of  either  the  irritation  or  the  movement;  the 
explanation  being  that  the  irritation  causes  an  impression  to 
be  transmitted  by  the  sensory  nerves  to  the  cord,  and  this 
is  thence  reflected  in  such  manner  down  the  motor  nerves 
as  to  produce  a  co-ordinated  contraction  of  the  muscles. 

The  same  points  are  still  better  illustrated  in  those 
instances  in  which  the  spinal  cord  has  been  injured  by 
violence  in  man,  so  as  practically  to  divide  it.  If,  in  such  a 
case,  the  soles  of  the  feet  are  tickled,  the  limbs  are  drawn 
up,  but  the  patient  is  able  to  tell  you  that  he  is  quite  uncon- 
scious of  the  whole  matter.  And  here  it  is  to  be  noticed 
that  reflex  movements  are  occasioned  much  more  easily  in 
such  a  case  than  in  healthy  circumstances,  when  the  com- 
munication with  the  brain  is  uninjured.  It  is  as  if  the  con- 
sciousness exercised  a  control  over  the  tendency  to  reflex 
action;  or  as  if  the  force  were  carried  on  to  the  brain, 
instead  of  being  reflected  back  by  the  motor  nerves.  So 
also,  reflex  action  takes  place  more  easily  during  sleep  than 
when  one  is  awake;  and  to  this  is  to  be  attributed  the  ease 
with  which,  during  sleep,  deep  parts  are  affected  by  irrita- 
tions applied  to  the  surface  over  them,  so  that  exposure  of 
the  chest  to  cold  during  sleep  is  more  dangerous  than  at 
other  times. 

When  one  side  of  the  spinal  cord  of  an  animal  is  divided  in 
the  dorsal  region,  sensation  is  lost  in  the  hind  limb  on  the 
opposite  side,  and  movement  on  the  same  side  as  the  opera- 
tion. When  a  longitudinal  division  is  made  down  the  middle, 
separating  one  lateral  half  of  the  cord  from  the  other,  in  the 
region  from  which  spring  the  nerves  to  either  fore  or  hinder 
limb,  sensation  is  destroyed  on  both  sides,  while  motion  is 
unaffected.  It  is  plain,  therefore,  that  the  tracts  through 
which  sensory  impressions  are  conducted  cross  the  middle 
line  soon  after  the  entrance  of  the  posterior  roots  of  nerves 
into  the  cord,  while  motor  impressions  pass  directly  down. 
But  immediately  above  the  spinal  cord,  in  the  fore  part  of 
the  portion  of  the  brain  termed  the  medulla  oblongata,  which 


THE  SPINAL  COBD.  191 

is  continuous  with  the  cord,  a  number  of  bundles  of  white 
fibres  are  seen  crossing  the  middle  line,  forming  what  is 
called  the  decussation  of  the  anterior  pyramids.  If  a  section 
be  made  in  the  middle  line,  longitudinally  through  that 
decussation,  it  is  followed  by  loss  of  all  voluntary  movement 
on  both  sides;  and  all  sections  on  one  side  of  the  brain 
above  that  point,  which  injure  the  power  of  movement,  do 
so  on  the  opposite  side  from  that  on  which  they  are  made. 
Therefore  the  tracts  by  which  motor  impulses  pass  from  the 
brain  to  the  muscles  cross  the  middle  line,  just  as  sensory 
impressions  cross,  and  each  side  of  the  brain  is  connected 
with  the  opposite  side  of  the  body,  as  regards  both  sensation 
and  motion;  but  the  sensory  decussation  takes  place  through- 
out the  spinal  cord,  and  the  motor  decussation  in  the  medulla 
oblongata. 

It  further  appears  that  the  conduction  of  sensory  impres- 
sions upwards  takes  place  by  means  of  the  grey  matter  of  the 
cord.  If  a  cut  be  so  made  as  to  divide  the  white  substance  of 
the  posterior  half  of  the  cord,  while  at  a  somewhat  different 
level  the  white  substance  of  the  anterior  half  be  divided,  care 
being  taken  to  leave  the  grey  matter  as  much  as  possible 
uninjured,  sensation  remains  unimpaired.  On  the  other  hand, 
if  an  instrument  be  introduced  with  as  little  damage  as 
possible  to  the  white  matter,  and  be  moved  so  as  to  divide 
the  whole  of  the  grey,  sensation  is  completely  destroyed 
below  the  site  of  the  injury.  Voluntary  movement,  as  well 
as  sensation,  continues  when  the  white  columns  have  been 
all  divided  and  the  grey  matter  has  been  left  intact;  but  if 
the  grey  matter  be  completely  divided,  together  with  the 
posterior  columns,  a  certain  amount  of  stimulus  to  voluntary 
movement  is  still  conveyed  through  the  anterior  and  lateral 
columns.  It  therefore  appears  that,  while  sensory  impres- 
sions are  conveyed  entirely  through  the  grey  matter,  motor 
impressions  are  conveyed  through  both  grey  and  white. 

143.  Another  point  in  the  physiology  of  the  spinal  cord  is 
worthy  of  mention.  When  the  posterior  columns  are  divided 
in  the  dorsal  region,  not  only  is  the  operation  in  itself  pain- 
ful, but  sensibility  becomes  inexplicably  exalted  in  the  parts 
supplied  from  beyond  the  lesion,  and  irritation  of  either  lip  of 
the  wound  gives  great  pain.  But  if  the  division  be  made  in 


192  ANIMAL   PHYSIOLOGY. 

the  neck,  where  the  origins  of  the  nerves  are  further  separ- 
ated; and  if  it  be  made  at  a  point  midway  between  the 
origins  of  successive  nerves,  not  only  is  it  painless,  but 
irritation  of  the  lips  of  the  wound  produces  no  pain.  From 
this  it  is  concluded  that  the  proper  fibres  of  the  cord,  those 
not  directly  continuous  with  the  nerves,  are  insensible  to 
irritation,  and  that  the  pain  in  the  dorsal  region  is  occasioned 
by  irritation  to  the  fibres  of  the  sensory  nerve-roots  as  they 
pass  in  to  the  grey  matter,  some  of  them  passing  upwards 
and  others  downwards  in  their  course  to  the  posterior  cornu, 
so  that  there  are  divided  fibres  in  connection  with  the  grey 
matter  in  both  the  lips  of  the  wound.  And  not  only  are  the 
fibres  proper  to  the  cord  insensible  to  direct  irritation,  but 
so  also  is  the  brain.  Slicing  the  brain  away  in  experiments 
on  animals  is  painless,  and  disease  of  the  substance  of  the 
brain  is  painless,  although  inflammation  of  the  membranes 
covering  it  is  acutely  painful,  the  membranes  having  nerves 
distributed  in  them.  There  is  thus  a  very  marked  difference 
in  the  irritability  of  the  fibres  of  the  central  nervous  system 
and  the  peripheral  nerves;  it  does  not,  however,  follow  that 
the  active  or  impressed  condition  is  not  of  the  same  descrip- 
tion in  both. 


CHAPTER  XIV. 

THE  NERVOUS  SYSTEM— Continued. 

144.  Structure  of  the  Encephalon. — The  brain  or  ence- 
phalon,  the  portion  of  the  cerebro-spinal  axis  contained  within 
the  cranium,  consists  of  various  parts  to  which  different  names 
are  given.  The  part  in  continuity  with  the  spinal  cord,  as  has 
already  been  mentioned,  is  called  the  medulla  oblongata.  It 
is  about  an  inch  and  a  quarter  long,  and  broadened  above,  and 
in  it  both  the  white  and  grey  matter  have  a  different  arrange- 
ment from  that  existing  in  the  cord.  In  front  are  the 
columns  called  anterior  pyramids,  whose  decussation  has 
already  been  mentioned;  outside  these  are  the  olivary  bodies, 
each  containing  a  grey  centre  of  unknown  function;  while 
outside  these  are  two  stout  pillars  called  restiform  bodies, 
including  the  posterior  half  of  the  lateral  columns  of  the 
cord,  and  all  the  fibres  of  the  posterior  columns  with  the 
exception  of  two  small  bands  behind,  distinguished  as  pos- 
terior pyramids.  These  and  the  restiform  bodies  slope  out- 
wards as  they  ascend,  and  limit  at  the  back  of  the  medulla 
oblongata  a  groove  lined  with  grey  matter,  continued  up 
from  the  central  canal  of  the  spinal  cord.  This  groove 
forms  the  floor  of  what  is  called  the  fourth  ventricle  of 
the  brain,  a  space  between  the  medulla  oblongata  and  the 
cerebellum. 

Above,  the  medulla  oblongata  is  crossed  in  front  by  a  thick 
transverse  band,  called  the  pons  Varolii,  which  conceals  the 
continuation  of  the  fibres  of  the  medulla  upwards  to  the 
cerebrum,  and  sends  its  fibres  into  the  cerebellum  on  each 
side.  Below  them  the  restiform  bodies  enter  the  cerebellum, 
and  thus  are  formed  what  are  called  the  middle  and  inferior 
crura  cerebelli;  while  the  superior  crura  are  a  pair  of  bands 
which  pass  from  the  cerebellum  to  the  cerebrum.  All  the 


194 


ANIMAL  PHYSIOLOGY. 


fibres  of  the  medulla  oblongata,  with  the  exception  of  the 
restiform  bodies,  pass  up  to  the  cerebrum. 

The  cerebellum    is  a 
large  mass  of  brain  sub- 
stance  covered   on    the 
surface  with  grey  matter 
arranged     in     complex 
transverse  laminae,  with 
folds  of  pia    mater  be- 
tween them.     Its  main 
bulk  in  the  human  sub- 
ject consists  of  two  -large 
lateral   lobes ;    but    be- 
tween these,  in  a  depres- 
sion or  vallecula  below, 
there  is  another  portion, 
the   inferior    vermiform 
process ;     while     above 
there    is    an    elevation 
where  the  lateral  lobes 
Fig.  99. — UNDER  SURFACE  OF  BRAIN,  meet  in  the  middle,  the 
a,  Spinal  cord  cut  across  below  the  superior  vermiform  pro- 
medulla  oblongata :  on  the  latter  are  -,       i     .       • 
seen,  from  the  middle  line  outwards,   CGSS>.  and  .what  S1V6S   a 
anterior    pyramids,    olivary    bodies,   special     significance     to 
and  restiform  bodies,    b,  Pons  Varolii ;  these   processes   is,   that 
c,  infundibulum  (the  pituitary  body  they   correspond  with  a 
having  been  removed) :  a.  ayn  operti,        •  /  77     77        /•   .7 
or  islLd  of  Reil;   e,   section  of  de-  middle   l?be  °f  the  cere' 
scending  cornu    of   the    left    lateral  bettum    in    other   mam- 
ventricle,    displayed    by   removal  of  mals,  which  in  the  lower 
part  of  the  middle  lobe  of  the  hemi-  orclers  forms  the  greater 
sphere,    exhibiting    sections    of    the          ,       f     ,1       e.iJri  «•*••,•,  ™* 
t^nia  hippocampi  and  hippocampus  Partn  ,°»     ^    staructure. 
major;   /,    cerebellum.     1,  Olfactory  In    birds    there    are  no 
bulb;  2-9,  successive  cranial  nerves,  lateral     lobes,     and     in 
marked  each  with  its  proper  number,  osseous  fishes  the  cere- 
bellum is  reduced  to  a  mesial  pouch,  without  lamination, 
whose  hollow  is  an  expansion  upwards  of  the  continuation  of 
the  original  central  canal.    In  the  human  subject  this  hollow 
is  seen  in  the  roof  of  the  fourth  ventricle,  in  front  of  the  in- 
ferior vermiform  process. 

The  medulla  oblongata,  pons  Yarolii,  and  cerebellum,  which 


STRUCTURE   OP   THE   ENCEPHALOtf, 


195 


may  be  termed  collectively  the  epencephalon  (Owen),  occupy 
an  inferior  compartment  in  the  back  part  of  the  cranium;  the 
cerebellum  being  roofed  in 
and  separated  from  the  rest 
of  the  brain  above  it  by  a 
septum  of  dura  mater 
called  the  tentorium  cere- 
belli,  which  in  some  ani- 
mals, as  the  cat,  even 
contains  a  lamina  of 
bone. 

145.  The  whole  of  the 
brain  above  the  level  of 
the  tentorium  is  included 
under  the  name  of  cere- 
brum, and  is  connected 
with  the  parts  below  by  a 
neck  or  isthmus,  in  thick-  Fig.  100.— MESIAL  SECTION  or  BRAIN. 
ness  about  the  size  of  a  «»  Medulla  oblongata ;  6, ,  pons  Va- 
n  i  •  i  .  roln  :  c,  fourth  ventricle,  and, 

florin,    which    traverses    a       Behind  it,  valve  of  Vieusseiis;  d, 

iter,  and,  behind  it,  corpora  quad- 
rigemina  ;  e,  pineal  body ;  /,  optic 
thalamus  looking  into  the  third 
ventricle,  and,  in  front  of  it,  an 
open  passage  from  the  third  to 
the  lateral  ventricle,  called  fora- 
men of  Monro ;  g,  left  layer  of 
septum  lucidum  bounding  the  fifth 
ventricle,  and,  beneath  it,  the 
foriiix  ;  h,  posterior  extremity  of 
corpus  callosum  above  the  trans- 
verse fissure ;  i,  optic  nerve ;  Jc, 
pituitary  body  ;  I,  one  of  the  cor- 
pora albicantia. 

The  cerebral  hemispheres  ill  man  form  by  far  the  most 
bulky  part  of  the  brain.  They  are  covered  with  a  thick 
coating  of  grey  matter  of  stratified  structure,  exhibiting  an 
arrangement  of  corpuscles  and  fibres  connecting  one  part  of 
it  with  another,  and  the  whole  with  the  parts  of  the  brain 
from  which  the  hemispheres  arise,  as  illustrated  in  fig.  101. 
This  grey  master  is  thrown  into  a  number  of  convolutions 
arranged  on  a  definite  plan,  though  varying  in  their  finer 


space  left  between  the  free 
edge  of  the  tentorium  and 
the  body  of  the  sphenoid 
bone.  This  isthmus,  as 
seen  from  below,  consists 
of  two  thick  pillars,  the 
crura  cerebri,  emerging 
above  the  poiis  "Varolii, 
diverging  as  they  ascend, 
and  almost  immediately 
concealed  by  the  two  cere- 
bral hemispheres. 


196 


ANIMAL   PHYSIOLOGY. 


details  in  different  individuals,  and  even  on  the  two  sides  of 
the  brain.  They  are  absent  in  the  lower  orders  of  mammals, 
become  more  abundant  the  higher 
we  ascend  in  the  series,  and  reach 
their  greatest  complexity  in  man. 
The  only  plausible  explanation  of 
the  use  of  these  convolutions  is  this, 
that  grey  matter  requiring  a  rich 
vascular  supply,  and  only  minute 
vessels  being  admissible  within  it, 
increase  of  surface  is  a  necessary 
condition  of  increase  of  bulk,  so  as 
to  bring  its  parts  sufficiently  near 
the  pia  mater,  in  which  the  arteries 
for  its  nourishment  divide.  Nor  is 
this  supposition  contradicted  by  the 
presence  of  convolutions  in  the  brains 
of  small  animals;  for  in  them  the 
length  of  capillary  travelled  by  the 
blood  in  each  circulation  is  likewise 
small,  and,  therefore,  the  distance 
to  which  the  blood  can  penetrate 
from  the  pia  mater  may  be  supposed 
to  be  in  proportion  to  the  size  of  tho 
animal. 

The  hemispheres  are  in  contact 
with  very  nearly  the  whole  extent  of 
the  cranial  wall  above  the  ten- 
torium.  They  are  separated  one 
from  the  other  above,  behind,  and  in 
front  by  a  deep  longitudinal  fissure, 
into  which  dips  a  process  of  dura 
mater,  called  the  falx  cerebri,  at- 
tached behind  along  the  middle  line 
of  the  tentorium,  and  in  front  to 
the  mesial  part  of  the  ethmoid  bone. 
At  the  bottom  of  the  longitudinal 

TV     TAT      n        TIT  fissure  they  are  united  by  a  thick 

Fig.  101.— GREY  MATTER   ,  J  1,11? 

OF  THE  CONVOLUTIONS:  transverse    commissure  or  bond   of 

ideal  vertical  section.       junction,  the  corpus  callosum,  which 


STRUCTURE  OF  THE  EHCE2HALON. 


19? 


presents  a  thick  posterior  border  in  front  of  the  tentorial 

attachment  of  the  falx,  and  at  its  fore  part  curves  downwards 

behind  the  ethmoidal 

attachment     of     that 

structure,    to   become 

continuous     with     a 

thin     lamina     which 

completes  the  floor  of 

the  cavity  of  the  brain 

in  the  middle  line. 

The  parts  of  the 
hemispheres,  which 
project  forwards,  rest- 
ing on  the  anterior 
fossa  of  the  skull,  over 
the  orbits,  are  called 
anterior  lobes ;  the 
parts  above  the  cere- 
bellum are  the  poste- 
rior lobes,  and  the 
parts  turning  down 
over  the  crura  cere- 
bri,  and  resting  in 
the  middle  fossae,  on 
the  sphenoid  and  tem- 
poral bones,  are  dis- 
tinguished as  the. 
middle  lobes. 

146.  If  a  human 
brain,  or  still  better, 
the  brain  of  any  of 
the  domestic  quadru- 
peds, be  examined, 
and  the  cerebellum 
be  turned  aside  from 


&'  102.— DIAGRAM  OF  BRAIN.  ^  «,  Spinal 
cord  ;  b,  b,  cerebellum,  divided  and, 
above  it,  the  valve  of  Vieussens  partially 
divided ;  c,  corpora  quadrigemiiia  ;  d,  dt 
optic  thalami ;  e,  pineal  body  ;  /,  /,  cor- 
pora striata ;  g,  g,  cerebral  hemispheres 
in  section;  7t,  corpus  callosum  ;  i,  fornix; 
I,  lf  lateral  ventricles  ;  3,  third  ventricle; 
4,  fourth  ventricle ;  5,  fifth  ventricle, 
bounded  on  each  side  by  septum  lucidum. 

the  hemispheres,  the  isthmus  will  be  brought  into  com- 
plete view,  and  there  will  be  displayed  above  it  an  eleva- 
tion divided  by  a  crucial  depression  into  four  parts,  and 
called  on  that  account  corpora  quadrigemina.  Two  tracts 
will  also  be  seen,  about  half  an  inch  separate,  adherent  to 


198  ANIMAti   PHYSIOLOGY, 

the  crura  cerebri,  and  proceeding  to  the  corpora  quadrigemiim 
from  the  cerebellum;  these  are  the  superior  crura  cerebelli 
already  alluded  to ;  and,  between  these,  a  thin  lamina,  called 
valve  of  Vieussens,  limited  by  the  cerebellum  behind  and  the 
corpora  quadrigemina  in  front,  forms  the  roof  of  the  fore  part 
of  the  fourth  ventricle,  as  that  hollow  is  continued  forwards 
into  a  narrow  canal  or  iter  which  passes  beneath  the  corpora 
quadrigemina,  and  opens  in  front  of  them. 

By  reflecting  the  hemispheres  well  forwards  off  the  crura, 
a  pair  of  large  elevations,  the  optic  thalami,  will  be  exposed 
in  front  of  the  corpora  quadrigemina ;  and,  by  dividing  the 
corpus  callosum  and  other  structures,  so  as  to  permit  more 
complete  reflection  of  the  hemispheres,  still  another  pair  of 
elevations,  the  corpora  striata,  will  be  seen  in  front  of  the 
optic  thalami,  and  external  to  them.  A  soft  body,  about  the 
size  of  a  pea,  the  pineal  body,  will  be  likewise  noticed 
attached  by  a  slender  connection  in  front  of  the  corpora 
quadrigemina,  and  overhanging  them,  imbedded  in  pia  mater. 
It  may  be  mentioned  of  this  structure  that  it  is  remarkable 
in  being  present  in  all  the  divisions  of  the  vertebrata, 
although  in  the  lower  forms  represented  by  little  else  than 
vascular  tissue,  and  in  man  consisting  of  degenerated  brain 
structure. 

147.  On  the  under  surface,  or  base  of  the  brain,  the  crura 
cerebri  are  crossed  by  two  bands  of  fibres,  which  can  be 
traced  round  the  crura  to  the  back  parts  of  the  optic  thalami, 
and  to  the  corpora  quadrigemina.  These  bands,  the  optic 
tracts,  meet  in  the  middle  line,  and  enclose  a  lozenge -shaped 
interval  between  the  crura ;  at  their  place  of  junction,  the 
optic  commissure,  they  exchange  fibres,  and  in  front  of  this 
they  diverge  as  the  optic  nerves,  or  nerves  of  sight,  to  the 
eyeballs.  In  the  lozenge-shaped  interval,  between  the  crura, 
are  seen  a  pair  of  white  bodies  like  small  peas,  the  corpora 
albicantia,  which  are  closely  connected  with  the -optic  thalami, 
and  in  front  of  them  a  funnel  of  membranous  grey  brain 
matter,  the  infundibulum,  leading  to  a  firm  body  which  lies 
in  the  sella  turcica  of  the  sphenoid  bone,  and  is  called  the 
pituitary  body — a  structure  certainly  not  nervous,  of  function 
quite  unknown,  but  well  developed  in  all  divisions  of  the 
vertebrata.  Outside  the  fore  part  of  the  lozenge-shaped 


STRUCTURE   OF   THE   ENCEPHALON. 


199 


interval,  on  each  side  lies  a  deep  fissure,  fissure  of  Sylvius, 
separating  the  middle  from  the  anterior  lobe  of  the  hemi- 
sphere, and  in  this  is  concealed  a  group  of  convolutions,  the 
island  of  Reil,  corresponding  exactly  in  position  with  the 
corpora  striata  seen  from  the  interior  of  the  brain.  On  the 
under  surface  of  the  anterior  lobes  are  the  olfactory  bulbs, 
from  which  the  nerves  of  smell  take  origin. 

148.  There  are  many  other  complications  in  the  structure  of 
the  brain  which  have  not  been  alluded  to;  but  probably  the 
best  general  conception  of  the  whole  organ  will  be  obtained 
by  looking  now  at  some  of  the  simpler  forms  seen  in  the  lower 
animals,  and  in  development. 

If  we  examine  the  brain  of  a  codfish,  we  see  at  the  back 
part  the  "  posterior"  columns  of  the  cord  separate  one  from 
the  other,  so  as  to  leave  a  hollow  between  them,  with  a 
mesial  groove,  which  is  continuous  with  the  central  canal  of 
the  cord,  and  is  the  fourth  ventricle.  Overlying  this  is  a 
mesial  pouch,  the  cerebellum;  in  front  of  the  cerebellum  are 
two  bodies,  the  optic  lobes,  likewise  hollow;  and  in  front  of  the 
optic  lobes  is  another  pair  of  small  bodies,  which  may  be  called 


Fig.  103.  —  BRAIN  OF  A  COD.  A,  From  above.  B,  From  below, 
a,  Medulla  ohlongata;  6,  cerebellum;  c,  optic  lobe;  d,  hemi- 
sphere-vesicle ;  e,  olfactory  bulb;  /, /,  optic  nerves;  g,  g,  hypo- 
aria  ;  h,  pituitary  body. 


200 


AtflMAL   PHYSIOLOGY. 


hemisphere-vesicles;  while  foremost  of  all  are  two  olfactory 
lobes  or  bulbs.  In  front  of  the  medulla  oblongata,  on  the 
under  surface,  are  two  masses 
placed  close  together  with  a  slight 
elevation  between  them,  the  hypo- 
aria  or  inferior  lobes,  and  imme- 
diately in  front  of  them  the 
pituitary  body.  The  hypoaria  lie 
beneath  the  optic  lobes;  and,  in 
front  of  them,  apparently  arising 
from  the  optic  lobes,  are  the  optic 
nerves,  which,  as  they  pass  for- 
wards, cross  one-  over  the  other, 
so  that  each  supplies  the  eye  of 
the  side  opposite  to  that  from 
which  it  arises. 

In  a  turtle's  brain  there  is  no 
difficulty  in  recognising  the  me- 
dulla oblongata,  cerebellum,  optic 
lobes,  and  olfactory  bulbs,  and  the 
olfactory  and  optic  nerves;  but 
the  hemisphere-vesicles  are  much 
larger  than  in  the  cod.  On  open- 
ing up  the  brain,  the  common 
ventricle,  prolonged  forwards  un- 
derneath the  cerebellum,  is  seen 
to  turn  downwards,  and  terminate 
in  a  cul-de-sac  at  the  pituitary 
body;  and,  above  this  point,  it 

T^,.       in/l        -n  bifurcates  to  extend  through  the 

Fig.     104.  —  BRAIN     OF    A  ,       .     ,  .  , 

TURTLE,  opened  along  the  hemisphere-vesicles  and  olfactory 
right  side,  a,  Cerebellum;  bulbs.  In  the  floor  of  each  hemi- 
&,  optic  lobe  ;  c,  hemi-  sphere- vesicle  is  a  thickened  part, 
sphere;  d  olfactory  bulb;  th  us  striatu'm.  and  in  the 

e,  pineal  body;/,  opening          .,       *      ,,  •  i      .  i 

into  infundibulum,  and,  to  <»?%  of  tn^  vesicle  there  is  a 
the  left  of  that,  the  choroicl  digitate  vascular  expansion  of  tho 
plexus  ;#,  corpus  striatum;  pia  mater,  the  choroid  plexus, 
h>  °Ptic  nerve-  which  enters  from  the  exterior 

at  a  spot  on  the  mesial  side  of  the  vesicle,  at  its   back 

part,  where  there  is  a  breach  of   continuity  in  the  brain 


STRUCTURE   OF   THE   ENCEPHALON. 


201 


matter  of  the  vesicular  wall.  There  is  also  another  vascular 
development  inwards  of  the  pia  mater,  between  the  cere- 
bellum and  the  medulla  oblongata,  the  choroid  plexus  of  the 
fourth  ventricle.  In  this  brain  we  miss  the  hypoaria  of  the 
brain  of  fishes;  but  it  may  be  noted  that,  close  to  the  position 
where  they  might  be  expected,  there  is  seen  in  the  interior  a 
thickening  at  the  sides  of  the  mesial  canal,  where -it  dips 
down  at  its  termination.  The  optic  nerves  at  their  decussa- 
tion  are  partially  blended  in  an  optic  commissure,  and  behind 
that  point  are  inseparable  from  the  brain,  and  named  optic 
tracts. 

In  the  brain  of  a  bird,  for  example  the  turkey,  the  cere- 
bellum is  no  longer  a  mere  hollow 
vesicle;  its  cavity  is  minute,  its 
surface  covered  with  grey  matter, 
and  thrown  into  deep  transverse 
laminae.  The  optic  lobes  project 
laterally,  and  even  downwards,  in- 
stead of  upwards.  There  is  a 
thorough  decussation  of  the  optic 
tracts.  The  hemisphere  -  vesicles 
when  opened  are  seen  to  be  covered 
with  a  very  thin  lamina,  and  filled  . 
up  by  the  projection  upwards  of  the  Fig.  105.  —  BRAIN  OF  A 
corpora  striata,  so  that  what  are  usu-  TURKEY.  a  Medulla 
,,  ,  ,,  ',  .  -,  r  i  •  i  oblongata  ;  &,  cerebel- 

ally  called  the  hemispheres  of  birds,      lum  .°c>  optic  lobe;  d, 
may  be  said  to  consist  principally  of 
corpora  striata.     Between  the  hemi- 
sphere-vesicles   and  the  optic  lobes 
there  is  a  thick  neck  of  substance,  but  110  very  important 
mass  deserving  a  name. 

149.  If  we  pass  now  to  a  mammalian  brain  in  a  foetal  stage, 
such  as  the  lamb's  brain  in  fig.  106,  we  have  little  difficulty 
in  recognising  corresponding  or  homologous  structures.  The 
medulla  oblongata  and  cerebellum  are  obvious.  In  front  of 
the  cerebellum  are  the  optic  lobes,  placed  superiorly  as  in 
reptiles  and  fishes,  and  constituting  the  structures  divided  in 
mammals  by  a  crucial  depression  into  four  parts,  and  called 
the  corpora  quadrigemina.  In  front  of  these  are  the  hemi- 
sphere-vesicles containing  the  corpora  striata;  but  a  little 


roof  of  hemisphere  re- 
flected to  show  corpus 
striatum;  e,  optic  nerve. 


202  ANIMAL   PHYSIOLOGY. 

care  suffices  to  turn  the  hemisphere-vesicles  outwards,  and 
then  come  into  view  between  them  the  optic  thalami,  in 
this  early  stage  united,  like  the  corpora  quadrigemina,  one 
with  the  other,  over  the  mesial  canal,  and  covered  with  pia 
mater.  Along  by  the  anterior  and  outer  margin  of  each 
of  the  optic  thalami,  a  fissure  is  seen  between  it  and  the 
hemisphere-vesicle,  at  which,  as  in  the  turtle,  a  large  choroid 
plexus  enters;  and  the  ruptured  margin  of  the  vesicle  has  its 
edge  turned  away  from  the  middle  line  into 
the  interior  of  the  cavity.  At  a  later  period 
this  everted  part  adheres,  across  the  middle 
line,  with  its  fellow  of  the  opposite  side; 
and  thus  is  formed  a  structure  peculiar  to 
mammals,  the  fornix,  separated  from  the 
corpora  quadrigemina  and  optic  thalami  by 
a  transverse  fissure,  containing  the  invagi- 
Fig.  106. — BRAIN  nated  part  of  the  pia  mater  supporting  the 
OF  EMBRYO  choroid  plexus,  and  called  the  velum  inter- 

IrAMB'      ^^  positum.     At  a  still  more  advanced  period 

above    and  the  r '         .  .  ,1,1         •,.  /* 

rio-ht  side,      a,  °*  embryonic  growth,  the  adjacent  surfaces 

Medulla  oblon-  of  th^  hemisphere-vesicles    become   joined 

gata;  b,  cerebel-  together  by  development  of  the  corpus  cal- 

lum;  c,  corpora  IQ          which  ig  united  posterioriy  to  the  back 
quadrigemina    :  '„  ..      p       .      .,     •          ,       r  i 

d,  optic  thalami;  Part  °*  tne  ibrnix,  then  arches  forwards  at  a 

el   right    hemi-  higher  level,  and  turns  downwards  in  front  so 

sphere  -  vesicle  as  to  enclose  a  mesial  space  above  that  body, 

reflected;/  cor-  The         ^  of  the  walls  of  the  vesicles  limit. 
pus      striatum.  \  .  .  ,  , . 

feound  the  open-  ln§  *^1S  mesial  space  continue  very  slender, 

ing  into  the  la-  and  constitute  the  septum  lucidum;  and  the 

teral     ventricle  space  itself  is  termed  the  fifth  ventricle.    The 

is  the  rudimen-  cavities  of  the  hemisphere-vesicles  are  called 

tarv  fornix.  rne  ,17.       7         .-TJI  i.  .1 

choroid  plexus,  *ne  6w*™  ventricles;  the  space  between  the 

which,   at  this  velum  interpositum  and  optic  thalami,  as 

period,    is    ex-  Well  as  between  these  bodies,  is  the  third 

ceedingly  large,  venfr{cle  •  fas  fourth  ventricle,  as  has  already 

nfoved.6  "  ^een  s*a*ec^  ^s  *ne  space  between  the  cere- 

bellum and  medulla  oblongata;  and  the 
canal  continued  forwards  from  this,  beneath  the  corpora 
quadrigemina,  is  called  the  iter  (a  tertio  ad  quartum  ven- 
triculum)  or  aqueduct  of  Sylvius. 


ST&UCTU&E  OF  THE  ENCEPHALON. 


203 


In  the  developed  brain,  the  fissures  into  the  lateral  ven- 
tricles, converted  by  the  adhesion  of  the  lateral  halves  of 
the  fornix  into  one  trans- 
verse fissure  as  pointed  out, 
extend  round  the  crura 
cerebri  to  the  extremity  of 
the  middle  lobe  of  the 
brain  at  the  inner  end 
of  the  fissure  of  Sylvius; 
and  the  extensions  of  the 
lateral  ventricles,  into 
which  they  open,  are  called  | 
the  descending  cornua;  the 
margin  of  the  fissure  in 
each  descending  cornu  is 
bounded  by  a  slender  pro- 
longation of  the  fornix, 
called  tcenia  hippocampi, 
because  it  lies  beside  a 
convexity  of  the  floor  of  Fig.  107.— THE  LATERAL  VENTRICLES. 
the  cornu,  the  hippocampus  a,  Posterior  border  of  corpus  callo- 

sum;  0,  front  of  corpus  callosum, 
and  below  it  fifth  ventricle,  bounded 
by  the  layers  of  the  septum  lucidum; 
c,  fornix,  and,  beyond  it,  the  cho- 
roid  plexus ;  d,  corpus  striatuin, 
and,  between  it  and  the  fornix,  a 
portion  of  the  optic  thalamus  ;  e, 
section  of  corpus  striatum;  /,  de- 
scending cornu  of  lateral  ventricle, 
with  hippocampus  major  in  its 
floor,  and  taenia  hippocampi  in 
front;  g,  hippocampus  minor  in  the 
floor  of  the  posterior  cornu. 


major:  the  posterior  and 
anterior  cornua  being  blind 
pouches  in  the  correspond- 
ing lobes.  In  front,  the 
fornix  dips  down  in  the 
form  of  a  couple  of  pillars 
in  front  of  the  optic 
thalami;  and  these  pillars, 
after  dipping  to  the  base 
of  the  brain,  and  forming 


the  corpora  albicantia,  twist 

upwards,  and  enter,  each  one,  the  optic  thalamus  of  its  own 

side.     The  fornix  is  thus  a  band  of  junction  between  the 

back  parts  of  each  hemisphere  and  the  corresponding  optic 

thalamus. 

In  leaving  this  difficult  subject,  it  is  necessary  to  point  the 
student's  attention  to  one  point  which  appears  to  have  escaped  the 
attention  of  anatomists,  but  which,  to  me  at  least,  is  pretty  obvious 
from  considerations  alluded  to  in  the  preceding  description,  namely, 


204  ANIMAL   PHYSIOLOGY 

that  the  hypoaria  of  fishes  correspond  with  the  optic  thalami  of 
mammals.  In  corroboration  of  this  view,  it  may  be  mentioned  that 
in  various  fishes  the  optic  nerves  arise  from  the  hypoaria  as  well  as 
from  the  optic  lobes  or  corpora  quadrigemina.  No  doubt  the  optic 
thalami  look  upwards,  and  the  hypoaria  downwards;  but  in  their 
first  development  the  optic  thalami  are  directed  downwards,  the 
embryonic  vesicle  from  which  they  are  derived  (p.  287)  being  turned 
directly  down;  and  in  the  position  of  the  optic  lobes  of  birds,  as 
compared  with  those  of  other  animals,  we  have  a  parallel  instance 
of  homologous  brain-masses  being  developed  in  the  adult  state  in 
different  directions.  My  excuse  for  mentioning  this  in  an  elementary 
work  is,  that  without  recognition  of  this  hitherto  unappreciated  point 
the  simplicity  of  the  brain  cannot  be  recognised. 

150.  Cranial  Nerves  (fig  99). — From  the  under  surface 
of  the  brain,  a  number  of  nerves  emerge  which  are  termed 
cranial.  They  differ  greatly  among  themselves,  both  in  size 
and  function,  and  are  variously  numbered  by  different 
anatomists.  But  none  of  the  methods  of  enumeration  have 
the  smallest  title  to  be  considered  scientifically  accurate;  for 
they  all  agree  in  attempting  to  reduce  to  a  linear  series 
structures  which  do  not  serially  correspond;  therefore  it  is 
well  to  be  guided  by  motives  of  convenience,  and  follow  the 
plan  generally  adopted  by  English  writers. 

The  first  pair,  the  olfactory  nerves,  devoted  to  the  sense  of 
smell,  are  brushes  of  exceedingly  soft  and  delicate  filaments, 
given  off  from  the  under  surface  of  the  olfactory  bulbs  already 
alluded  to  as  lying  beneath  the  anterior  lobes  of  the  cerebral 
hemispheres.  These  olfactory  bulbs  are  much  more  largely 
developed  in  the  majority  of  mammals  than  in  man,  and  in 
some  of  them  have  cavities  communicating  with  the  interior 
of  the  brain.  They  are,  in  fact,  vesicular  outgrowths  from 
the  brain. 

The  second  pair,  the  optic  nerves  or  nerves  of  sight,  come 
off,  as  lias  been  already  explained,  from  the  optic  commissure. 
Development  shows  that  they  also  are  vesicular  outgrowths 
from  the  brain. 

The  third,  fourth,  and  sixth  pairs  are  all  of  them  small 
motor  nerves,  which  supply  muscles  of  the  eyeball,  the  third 
and  fourth  appearing  above  the  pons  Varolii,  and  the  sixth 
below  it. 

Thejffik  pair,  the  trifacial  nerves,  are  large  trunks,  whose 
fibres  take  origin  in  the  medulla  oblongata,  and  pierce  the 


CEANIAL   NERVES.  205 

pons  Varolii.  They  resemble  the  spinal  nerves,  in  consisting 
each  of  a  motor  and  sensory  root;  and  in  the  sensory,  which 
is  much  the  larger,  having  a  ganglion  on  it.  The  sensory 
part  separates  into  three  divisions,  and  supplies  the  scalp  in 
front  of  the  ear,  and  all  the  face,  as  well  as  the  teeth  and  a 
large  part  of  the  tongue,  with  sensation;  the  motor  part 
mixes  with  the  third  division  of  the  sensory,  and  is  distri- 
buted to  the  muscles  of  mastication. 

The  seventh  pair  consists  of  two  portions,  which  emerge 
on  the  surface  of  the  brain,  in  the  angle  between  the  crus 
cerebri,  the  cerebellum,  and  the  medulla  oblongata,  and  enter 
the  temporal  bone  together.  One  portion,  the  portio  mollis 
or  auditory  nerve,  is  the  nerve  of  hearing,  and  is  distributed 
within  the  bone;  the  other,  the  portio  dura  or  facial  nerve, 
is  conducted  through  the  temporal  bone,  and  proceeds  to  the 
face  to  supply  the  muscles  of  expression.  Within  the  tem- 
poral bone,  the  portio  dura  gives  off  the  chorda  tympani, 
which  traverses  the  tympanic  cavity,  and  joining  with  the 
lingual  branch  of  the  fifth  nerve  passes  to  a  small  ganglion, 
the  submaxillary  ganglion,  and  is  the  nerve  alluded  to  at 
p.  64  as  governing  the  secretion  of  the  submaxillary  gland. 

The  eighth  pair  comprises  three  pairs  of  trunks,  which  arise 
from  the  medulla  oblongata,  and  leave  the  skull  by  one  pair 
of  apertures.  (1.)  The  glossopharyngeal  nerves  supply  sensory 
branches,  devoted  to  the  sense  of  taste,  to  the  back  of  the 
tongue,  and  motor  and  sensory  branches  to  the  pharynx. 
(2.)  The  pneumogastric  or  vagus  nerve  is  both  sensory  and 
motor;  it  descends  on  the  oesophagus  to  the  stomach,  and 
filaments  may  be  traced  from  it  even  to  the  viscera.  It 
supplies,  in  its  course,  branches  to  the  pharynx,  larynx 
and  lungs,  and  the  heart,  and  is  intimately  associated  with 
the  sympathetic,  in  connection  with  which  it  will  be  again 
referred  to.  (3.)  The  spinal  accessory  is  entirely  motor,  and 
consists  of  two  parts,  of  which  one,  the  accessory,  joins  the 
pneumogastric,  while  the  other  is  distributed  to  two  large 
muscles,  the  sterno-mastoid  and  trapezius. 

The  ninth  pair,  or  hypoglossal  nerves,  are  the  motor  nerves 
of  the  tongue.  They  emerge  from  the  medulla  oblongata 
behind  the  olivary  eminences,  which  separate  them  from  the 
eighth  pair  in  front  of  these  eminences. 


206  ANIMAL   PHYSIOLOGY. 

151.  Functions  of  the  Encephalon.  —  The  medulla 
oblongata  is  principally  remarkable  for  its  connection  with 
respiration.  Respiration  is  a  reflex  act  in  which,  a  stimulus 
is  apparently  furnished  by  the  unaerated  blood,  an  impression 
is  conveyed  to  a  nervous  centre,  and  an  impulse  proceeds 
thence,  producing  a  co-ordinated  movement  of  the  muscles  of 
the  chest.  No  doubt  these  movements  are  capable  of  con- 
siderable control  by  the  will,  but  they  are  continued  in  con- 
ditions of  unconsciousness;  and,  although  by  an  effort  they 
may  be  delayed  for  a  moment,  the  impulse  soon  becomes 
imperative,  and  breaks  through  all  restraint.  The  centre 
engaged  in  this  reflex  action  is  the  medulla  oblongata;  and 
that  part  of  the  brain  is,  therefore,  of  the  utmost  importance 
for  the  continuance  of  life.  The  whole  of  the  rest  of  the 
brain  may  be  gradually  removed  without  any  interference 
with  respiration;  also  the  spinal  cord  may  be  divided  at 
different  levels;  and  only  when  the  section  is  made  high  up 
in  the  neck,  above  the  origin  of  the  (phrenic)  nerves  which 
supply  the  diaphragm,  is  respiration  materially  affected;  but 
when  that  portion  of  the  medulla  oblongata  is  removed  from 
which  the  vagus  nerves  take  origin,  respiration  ceases  at 
once,  and  the  animal  dies.  This  does  not  arise  from  mere 
interference  with  the  functions  of  the  vagus  ;  for  that  pair  of 
nerves  may  be  divided,  and  respiration  continues,  although, 
no  doubt,  the  entrance  of  air  into  the  windpipe  is  interfered 
with  by  paralysis  of  the  larynx,  and  even  when  that  incon- 
venience is  remedied  by  an  artificial  opening  into  the  trachea, 
death  results  after  a  time  from  the  irritation  of  foreign  bodies 
entering  the  lungs.  The  sudden  death  which  follows  removal 
of  the  upper  part  of  the  medulla  oblongata  is,  therefore,  only 
to  be  accounted  for  by  that  part  being  the  centre  from  which 
the  respiratory  movements  receive  their  impulse.  It  is  like- 
wise the  centre  engaged  in  the  act  of  swallowing,  which,  like 
respiration,  continues  to  be  performed  after  removal  of  the 
rest  of  the  brain. 

Being  the  centre  which  governs  respiration,  the  medulla 
oblongata  is  likewise  to  be  regarded  as  the  centre  engaged  in 
various  spasmodic  actions  of  an  occasional  kind.  In  cough- 
ing, an  irritation  of  the  pneumogastric  nerve  excites  first  a 
spasmodic  closure  of  the  glottis,  and  afterwards  a  convulsive 


FUNCTIONS  OP  THE  ENCEPHALON.         207 

expiration,  by  which  the  air  iorces  its  way  out  at  tlie  con- 
tracted opening.  In  sneezing,  an  irritation  of  the  fifth  nerve 
leads  to  a  convulsive  expiration  with  the  glottis  open,  but 
the  tongue  raised  so  as  to  divert  the  expelled  air  from 
escaping  by  the  mouth,  and  send  it  through  the  nostrils.  In 
hiccough,  the  glottis  is  shut,  and  a  momentary  spasmodic 
contraction  of  the  diaphragm  and  abdominal  walls  takes 
place ;  while  in  vomiting,  a  similar  action  is  more  prolonged. 
The  walls  of  the  stomach  appear  to  take  no  part,  or  only  a 
secondary  part  in  vomiting;  for  a  dog,  in  which  the  stomach 
was  replaced  by  a  bladder,  was  made  to  vomit  perfectly,  by 
injection  of  tartar  emetic  into  its  veins  (Magendie).  The 
excitation  of  the  reflex  action  in  vomiting  is  not  always 
the  same,  for  it  may  be  the  result  of  irritation  of  the  fauces, 
or  may  be  induced  by  nausea,  an  ill-understood  sensation 
depending  on  disturbance  of  the  cerebral  circulation. 

It  may  be  furthur  mentioned  that  irritation  of  the  medulla 
oblongata  in  the  floor  of  the  fourth  ventricle,  produces 
artificial  diabetes,  that  is  to  say,  sugar  in  the  urine.  This  it 
does  by  paralysing  the  blood-vessels  of  the  liver,  and  so  lead- 
ing to  an  abnormal  amount  of  sugar  being  thrown  into  the 
blood. 

152.  When  one  of  the  crura  cerebri  or  optic  thalami  is  divided 
or  destroyed,  total  paralysis  of  both  sensation  and  voluntary 
movement  is  the  result.  When  the  cerebellum  is  removed, 
the  power  of  standing,  and  of  all  steady  and  definite  move- 
ment is  lost,  although  the  animal  continues  to  move  its  limbs 
in  its  attempts  to  stand  and  walk.  It  seems  as  if  the  impulse 
to  voluntary  movement  descended  from  the  optic  thalami, 
while  the  power  of  co-ordination  of  movements  resided  in 
the  cerebellum. 

When  the  corpora  quadrigemina  are  destroyed,  total  blind- 
ness results;  and  when  only  one  side  is  destroyed,  there  is 
blindness  in  the  opposite  eye,  a  result  to  be  accounted  for  by 
the  crossing  of  fibres  in  the  optic  commissure.  It  is  curious 
that  injury  to  the  optic  thalami  appears  to  have  no  effect  on 
vision,  although  the  optic  tracts  arise  in  part  from  those 
bodies,  as  well  as  from  the  corpora  quadrigemina. 

With  regard  to  the  corpora  striata,  experiment  gives  none 
but  negative  evidence,  while  the  study  of  development  and 


208  ANIMAL  PHYSIOLOGY. 

comparative  anatomy  show  them  to  be  properly  considered 
as  part  and  parcel  of  the  cerebral  hemispheres. 

It  has  already  been  pointed  out  that  in  early  development 
the  corpora  striata  make  their  appearance  in  the  floor  of  the 
hemisphere-vesicles ;  they  are  covered  with  grey  matter  on 
the  surface,  continuous  with  that  which  lines  the  whole 
cylinder  of  the  cerebro-spinal  axis;  and  they  have  other 
patches  of  grey  matter  within  them,  which,  when  cut  across, 
present  the  striated  appearance  from  which  the  bodies  are 
named;  and  the  lowest  of  these  is  in  communication  with  the 
island  of  Reil,  so  that  a  communication  is  here  established 
between  the  grey  matter  lining  the  cerebro-spinal  canal,  and 
that  of  the  cerebral  convolutions.  We  have  seen  also  that 
in  different  animals,  while  the  corpora  striata  and  cerebral 
hemispheres  are  intimately  connected,  they  are  very  variously 
proportioned  one  to  the  other;  for,  in  fishes,  one  pair  of 
structures  represents  both;  in  the  turtle  a  small  corpus 
striatum  lies  at  the  bottom  of  each  hemisphere,  looking  into 
the  interior  of  its  vesicle;  and  in  birds,  the  islands  of  Keil 
and  corpora  striata  form  the  greater  part  of  the  hemispheres: 
indeed,  in  the  common  fow],  the  hemispheres  consist  of 
scarcely  anything  else;  and  when  a  physiologist,  in  vivi- 
section, slices  what  he  terms  the  hemispheres  from  a  fowl,  he 
in  reality,  removes  in  the  upper  slices  the  corpora  striata 
covered  with  a  thin  membrane  representing  the  roof  of  the 
hemispheres. 

153.  The  cerebral  hemispheres  are  the  parts  of  the  brain 
connected  with  the  higher  operations  of  intelligence.  The 
experiment  just  alluded  to,  as  performed  on  fowls,  can  be 
performed  less  easily  on  mammals;  but  the  result  in  both 
cases  is  the  same,  namely,  that  the  larger  the  part  of  the 
hemispheres  taken  away,  the  less  intelligence  remains.  The 
effect  of  removal  of  the  hemispheres  from  a  pigeon  is  gra- 
phically described  by  Dalton,  the  American  physiologist. 
"  The  bird  remains  sitting  motionless  on  his  perch,  or  stand- 
ing upon  the  ground,  with  the  eyes  closed,  and  the  head 
sunk  between  the  shoulders.  Occasionally,  the  bird  opens  his 
eyes  with  a  vacant  stare,  stretches  his  neck,  perhaps  shakes 
his  bill  once  or  twice,  or  smooths  down  the  feathers  upon  his 
shoulders,  and  then  relapses  into  his  former  apathetic  con- 


JUNCTIONS  OF  THE  fcxcEPHALosf.  209 

clitlon.  This  state  of  immobility,  however,  is  not  accompanied 
by  the  loss  of  sight,  of  hearing,  or  of  ordinary  sensibility. 
All  these  functions  remain,  as  "well  as  that  of  voluntary 
motion.  If  a  pistol  be  discharged  behind  the  back  of  the 
animal,  he  at  once  opens  his  eyes,  moves  his  head  half  round, 
and  gives  evident  signs  of  having  heard  the  report;  but  he 
immediately  becomes  quiet  again,  and  pays  no  further  atten- 
tion to  it.  ...  Longet  has  found  that  by  moving  a 
lighted  candle  before  the  animal's  eyes  in  a  dark  place,  the 
head  of  the  bird  will  often  follow  the  movements  of  the 

candle  from  side  to  side,  or  in  a  circle The 

limbs  and  muscles  are  still  under  the  control  of  the  will ;  but 
the  will  itself  is  inactive,  because,  apparently,  it  lacks  its 
usual  mental  stimulus  and  direction." 

It  is  difficult,  however,  to  say  how  far  that  part  of  the 
nervous  system  extends  on  which  the  existence  of  conscious- 
ness is  dependent.  Possibly  it  reaches  over  a  greater  area 
in  the  lower  than  the  higher  animals.  A  decapitated  frog 
can  be  made  to  leap,  and  will  thrust  objects  aside  when 
irritated;  and  although  these  movements  are  sometimes  said 
to  be  reflex,  it  is  not  easy  to  understand  how  they  can  be 
so.  But,  undoubtedly,  all  the  higher  manifestations  of  con- 
sciousness which  constitute  intelligence,  depend  on  the 
cerebral  hemispheres.  On  comparing  different  kinds  of 
mammals,  it  is  found  that  increased  development  of  the 
hemisperes  and  complexity  of  the  convolutions  into  which 
their  surface  is  thrown  are  associated  with  increased  intelli- 
gence. In  rodent  animals,  such  as  rabbits,  the  hemispheres 
are  small  and  smooth,  while  in  apes,  they  are  proportionally 
larger,  and  are  more  highly  convoluted  than  in  any  animal 
but  man.  Even  in  the  higher  races  of  men,  the  convolutions 
are  more  complex  than  in  the  lowest.  The  circumstance 
that  disease  of  the  grey  matter  of  the  hemispheres  is  liable 
to  be  accompanied  with  intellectual  derangement,  likewise 
points  to  a  connection  between  the  hemispheres  and  intelli- 
gence. 

But  if  the  sensorium,  or  seat  of  consciousness,  be  confined 

to  the  encephalon,  the  question  arises :  How  do  we  become 

aware  of  impressions  made  at  the  surface  of  the  body?     The 

old  physiologists  believed  in  the  diffusion  of  consciousness 

H  o 


210  ANIMAL   PHYSIOLOGY. 

through,  a  sensorium  commune  extending  throughout  the 
nervous  system;  but  the  loss  of  all  sensation  in  parts  whose 
nervous  communication  with  the  brain  has  been  severed  puts 
an  end  to  that  theory.  At  the  present  day,  it  is  customary 
to  say  that  the  mind  refers  impressions  received  at  the  brain 
to  the  extremities  of  the  nerves  by  which  they  have  been  con- 
ducted. But  it  is  perfectly  certain  that  there  is  no  separate 
nervous  communication  between  the  brain  and  each  point  of 
the  surface  of  the  body  in  which  sensations  can  be  dis- 
tinguished. The  question  is  of  the  utmost  interest  psycho- 
logically, but  is  still  unsettled.  Personally,  I  believe  that 
the  only  tenable  theory  yet  put  forward  is  that  which  I  have 
elsewhere  broached,  viz.,  that  while  consciousness  is  dependent 
on  the  encephalon,  the  sensorium  extends  thence,  so  far  as 
there  is,  at  any  moment,  unbroken  continuity  of  nerves  in 
the  active  or  impressed  condition. 

154.  It  is  frequently  supposed  that  different  parts  of  the 
hemispheres  are  connected  with  different  faculties  of  the 
mind;  and  the  opinion  that  the  frontal,  parietal,  and  occipital 
regions  are  devoted  respectively  to  the  intellectual  powers, 
moral  faculties  and  appetites,  or  in  some  other  way  differ 
in  function,  is  not  confined  to  the  believers  in  the  system 
founded  by  Gall,  and  commonly  known  as  phrenology.  But 
there  is  no  foundation  for  any  such  supposition.  On  the 
contrary,  the  evidence  points  to  an  opposite  conclusion. 
Serious  damage  to  the  hemispheres,  of  a  limited  description, 
often  occurs  without  loss  of  life,  and  such  cases  may  occur 
without  any  apparent  interference  with  intellectual  functions. 
A  tumour  may  press  on  some  particular  part  of  a  hemisphere 
without  producing  any  disturbance,  and  bodies  have  pene- 
trated the  brain,  and  portions  of  brain  protruding  from 
wounds  have  been  removed  by  surgeons,  without  any  obvious 
impairment  of  the  mental  faculties.  Not  only,  so,  but  the 
kind  of  disturbance  which  is  liable  to  occur  from  such  injuries 
does  not  vary  according  to  the  site  of  lesion,  so  as  to  affect, 
in  one  instance,  the  faculties  of  perception,  in  another  the 
disposition,  and  in  a  third  the  powers  of  volition,  as  might 
be  expected,  were  phrenological  theories  true. 

In  recent  years  a  little  colour  has  seemed  to  be  given  to 
the  theory  of  separate  organs  in  the  cerebral  hemispheres,  by 


FUNCTIONS  OF  THE  ENCEPHALON.          211 

the  discovery  that  imperfections  of  speech,  constituting  a 
condition  called  aphasia,  arise  from  disease  of  a  limited  por- 
tion of  the  brain,  immediately  outside  the  island  of  Reil, 
particularly  on  the  left  side.  In  these  cases  the  affections  of 
speech  are  very  various,  but  always  depend  on  mental  defi- 
ciency and  not  on  paralysis  of  the  tongue.  In  some  instances 
there  is  total  dumbness,  in  others,  incapability  of  clearly 
uttering  any  word;  and  in  a  larger  number  of  cases,  certain 
words  and  phrases  are  pronounced  perfectly,  but  they  are 
not  the  words  which  convey  the  idea  which  the  patient 
wishes  to  express.  Perhaps  some  word  is  repeated  on  all 
occasions;  and  even  when  the  right  expression  is  suggested 
to  the  patient,  he  is  unable  to  employ  any  other  than  that 
which  he  keeps  repeating,  although  quite  conscious  of  his 
blunder.  The  very  variety,  Lowever,  of  these  cases  of 
aphasia,  shows  that  they  do  not  arise  from  damage  to  the 
organ  of  a  specific  faculty.  It  is  more  rational  to  recognise 
in  them  the  result  of  a  lesion  which  interferes  with  the 
consentaneous  action  of  the  different  parts  of  the  hemisphere, 
by  attacking  the  fibres  where  they  emerge  from  the  corpus 
striatum  to  proceed  to  every  part.  And  we  may  well  believe 
that  consentaneous  action  of  the  hemispheres  is  especially 
required  in  so  complex  a  process  as  conversation,  which 
requires  a  number  of 'distinct  mental  operations  to  be  carried 
on  at  one  time. 

155.  A  consideration  of  the  different  mental  operations  re- 
quired in  talking,  is  exceedingly  instructive.  The  attention 
of  the  speaker  is  directed  principally  to  the  idea  which  he 
wishes  to  express.  And,  if  engaged  in  a  continuous  dis- 
course, he  must  at  the  same  time  think  of  the  sequence  of  his 
utterances,  and  especially  what  is  immediately  to  succeed  the 
sentence  with  which  he  is  engaged.  The  choice  of  words 
will  also  occupy  his  mind  to  a  recognisable  degree;  but  apart 
from  this  intentional  choice,  there  is  the  choice  of  the  simpler 
grammar  and  names  of  objects,  which  are  so  habitual  that 
we  fail  to  separate  them  in  our  thoughts  from  the  ideas 
which  they  express.  Still  less  attention  is  devoted  to  the 
complex  movements  of  the  organs  of  voice  and  speech; 
although  they  are  all  performed  in  the  service  of  the  mind, 
and  it  was  with  mental  effort  in  infancy  that  we  learned,  by 


212  ANIMAL   PHYSIOLOGY. 

means  of  observation  and  imitation,  to  accomplish  them. 
Gesture  also  accompanies  speech  without  attention  being  di- 
rected to  it;  and,  except  in  exceedingly  rare  cases  of  mental 
absorption,  the  speaker  during  all  these  mental  actions  is 
able  to  note  what  is  going  on  around  him. 

The  exactitude  of  the  tongue  in  speech  furnishes  but  one 
of  many  instances  of  complex  movements  performed  under 
mental  stimulus,  without  perceptible  attention  being  given 
to  them;  the  movements  of  the  limbs  in  walking  afford 
another  example;  and  these  are  the  kinds  of  acts  which  are 
sometimes,  although  erroneously,  called  unconscious  cerebra- 
tion. They  are  precisely  like  other  voluntary  movements, 
only  the  effect  of  habit  on  the  mind  is  such  that  the  mind 
gives  the  stimulus  to  the  nervous  system  to  accomplish  them, 
without  expenditure  of  attention.  But  the  mind  not  only 
may  initiate  commands  without  devoting  attention  to  them; 
it  may  receive  impressions  in  like  manner;  and  it  often 
happens  that  an  impression  received  without  perceptible 
attention  will  lead  to  a  customary  act.  Thus,  a  rider,  with- 
out conscious  effort,  accommodates  himself  to  the  movements 
of  his  horse,  and  a  sailor  balances  himself  on  board  a  ship. 
An  act  so  performed  may  well  be  called  automatic;  but  the 
term  acquired  reflex  action  sometimes  given  to  it  is  of  more 
doubtful  propriety;  for  in  true  reflex  action  there  is  an 
unbroken  sequence  of  physical  changes,  while,  in  such  actions 
as  these,  a  physical  cause  produces  a  psychical  effect,  and 
psychical  change  is  the  stimulus  to  the  movement. 

156.  The  most  moderate  exercise  of  the  mental  faculties, 
the  mere  continuance  of  consciousness,  appears  to  involve 
exhaustion  of  the  brain,  and  necessitates  restoration  of  its 
vigour  by  sleep.  Of  the  physical  relations  of  sleep  very  little 
is  known.  It  has  been  pointed  out  that  the  circulation  in 
the  brain  is  less  active  during  sleep  than  at  other  times,  but 
this  is  not  proved  to  be  constant;  nor,  supposing  it  to  be 
so,  does  it  sufficiently  explain  the  state  of  unconsciousness. 
It  may,  however,  be  fairly  assumed  that  the  passage  of  the 
brain  into  a  condition  of  inactivity,  is  the  cause  of  the 
cessation  of  mental  action.  Just  as  some  muscles,  for  ex- 
ample the  fibres  of  the  heart,  move  ceaselessly,  while  others 
require  rest,  so  some  of  the  nervous  centres,  including  the 


FUNCTIONS    OF   THE    ENCEPHALOIST.  213 

ganglia  which,  immediately  govern  the  heart,  are  in  con- 
tinual action,  while  others,  including  the  cerebral  hemi- 
spheres, require  considerable  pauses  for  the  renewal  of  their 
activity. 

Dreams,  like  sleep,  are  only  imperfectly  understood.  Their 
main  peculiarity  consists  in  a  certain  amount  of  mental 
activity  existing,  with  complete  or  almost  complete  cessation 
both  of  impressions  from  the  organs  of  sense,  and  of  voli- 
tionary  impulses  to  the  muscles.  In  these  circumstances, 
the  pictures  of  memory  and  imagination  come  into  the  fore- 
ground, unrepressed  by  the  stronger  representations  of  sense, 
and  assume  the  appearance  of  reality.  One  idea  suggests 
another,  and  each  one  which  is  sufficiently  vivid  has  in  turn 
the  semblance  of  reality;  and  from  this  arise  the  strange 
shifting  of  scenes  and  curious  confusions  with  which  every 
one  is  familiar. 

Apparitions,  and  other  illusions  from  mental  causes,  are  to 
be  accounted  for  in  a  similar  way.  Many  well  authenticated 
instances  are  on  record  of  figures  appearing  to  persons  other- 
wise perfectly  sane.  Among  them  may  be  mentioned  the 
case  of  Nicolai,  the  Berlin  bookseller,  who  saw  persons  in 
the  room  with  him,  when  he  knew  that  there  was  in  reality 
no  one  present;  but  so  far  from  being  disturbed  by  these 
apparitions,  made  them  the  subject  of  study  and  recorded 
the  details.  In  his  case,  they  were  traced  to  the  neglect 
of  a  customary  bloodletting,  and  disappeared  after  leeching. 
In  such  rare  occurrences  there  is  the  same  prominence  of  a 
mental  picture  as  occurs  in  dreams;  but,  in  dreams,  that 
prominence  results  from  the  absence  of  sensations  origi- 
nated by  contact  with  the  outer  world;  while,  in  appari- 
tions, it  is  the  consequence  of  some  pathological  action  within 
the  brain. 

In  somnambulism,  the  mind  is  likewise  occupied  as  in  a 
dream;  but  the  ideas  which  possess  it,  while  others  have  been 
excluded,  become  so  strong  that  the  apparatus  of  voluntary 
movement  and  of  the  senses  are  thrown  into  action  in  an 
automatic  fashion,  the  attention  being  directed  to  the  all 
absorbing  imagination.  The  'mesmeric  trance  is  a  very  similar 
condition,  in  which  the  will  is  altogether  governed  by  the 
ideas  impressed  by  another  person. 


214  ANIMAL   PHYSIOLOGY. 

157.  The  Sympa- 
thetic System.  —  This 
is  the  portion  of  the 
nervous  system  by  which 
the  viscera  are  princi- 
pally supplied.  The 
primary  part  consists  of 
two  chains  of  ganglia, 
one  on  each  side,  in 
front  of  the  vertebral 
column,  called  the  pre- 
vertebral  chains,  or  the 
great  sympathetic. 

In  the  dorsal,  lumbar, 
and  sacral  regions,  these 
chains  present  a  ganglion 
for  almost  every  spinal 
nerve,  and  each  spinal 
nerve  has  a  twig  of 
communication  with  its 
corresponding  ganglion. 
Inf  eriorly  the  two  chains 
meet  together  in  a  gan- 
glion impar  in  front  of 
the  coccyx.  In  the  neck 

Fig.  108.  —  SYMPATHETIC 
SYSTEM  OF  NERVES,  a, 
Superior  cervical  gan- 
glion, from  which  the 
sympathetic  chain  is  con- 
tinued regularly  down- 
wards as  far  as  the  coc- 
cyx, where  it  communi- 
cates with  its  fellow.  It 
is  likewise  continued  ir- 
regularly in  the  cranium. 
At  b,  the  chains  of  oppo-^ 
site  sides  communicate* 
behind  the  upper  incisors; 
c,  cardiac  plexus;  d,  solar 

plexus  ;  e,  hypogastrlc  plexus;  /,  renal  and  supra-renal  plexus.  1 
and  2,  First  and  second  divisions  of  fifth  cranial  nerve  ;  3,  vagua 
nerve;  4,  first  spinal  nerve. 


THE   SYMPATHETIC  SYSTEM.  215 

there  are  only  three  ganglia,  but  they  communicate  with 
all  the  cervical  nerves;  and  the  uppermost  ganglion,  which  is 
the  largest,  sends  branches  upwards  into  the  skull,  round 
the  internal  carotid  artery.  Within  the  skull  the  chain 
can  be  traced,  in  somewhat  irregular  fashion,  communi- 
cating with  the  fifth  and  other  nerves;  and  two  cords  pass 
forwards,  one  on  each  side  of  the  septum  of  the  nose,  to 
unite  on  the  palate  behind  the  incisor  teeth,  and  form  the 
superior  termination  of  the  chain,  at  the  spot  which  I  once 
demonstrated,  and  still  hold,  to  be  the  arch  of  the  foremost 
segment  of  the  skull.  , 

In  the  neck,  the  sympathetic  chains  give  off  branches  to 
the  heart;  and  in  the  chest  they  send  twigs  to  the  lungs. 
From  the  thoracic  ganglia  there  likewise  descend  three  pairs 
of  splanchnic  nerves,  which  form  a  very  large  plexus  in  the 
upper  part  of  the  abdomen,  the  solar  plexus.  This  plexus 
contains  two  large  semilunar  ganglia,  and  sends  branches 
along  the  blood-vessels  to  the  stomach,  liver,  intestines, 
and  other  abdominal  viscera,  and  communicates  by  large 
branches  on  the  aorta  with  the  hypogastric  plexus, 
which  is  placed  within  the  pelvis  and  supplies  the  viscera 
there. 

The  sympathetic  system  is  especially  devoted  to  the  supply 
of  the  viscera  and  blood-vessels,  but  it  is  by  no  means  inde- 
pendent of  the  cerebro-spinal  system;  as  is  proved  anatomi- 
cally by  its  close  connection  with  the  pneumogastric  nerves 
and  its  communications  with  the  spinal  cord,  and  physiologi- 
cally by  the  conveyance  of  influences  through  it  from  the 
cerebro-spinal  axis.  That  nervous  influence  is  so  conveyed 
is  illustrated  by  many  familiar  effects  of  mental  conditions 
on  different  visceral  actions,  but  still  more  explicitly  by 
the  effects  of  experiments  on  the  nerves  to  the  blood- 
vessels. 

158.  The  vaso-motcr  nerves,  or  nerves  to  the  arterioles,  form 
an  important  part  of  the  sympathetic  system.  When  in  action, 
they  contract  the  muscular  coats  of  the  vessels,  and  limit  the 
amount  of  blood  to  the  part  supplied  by  them;  and  it  has 
been  already  pointed  out  (p.  125),  that  division  of  the  sym- 
pathetic in  the  neck  causes  paralysis,  and  consequent  disten- 
sion, of  the  arteries  of  the  head.  But  division  of  the  spinal 


216  ANIMAL  PHYSIOLOGY. 

cord  in  the  cervical  region  produces  paralysis  of  the  blood- 
vessels of  the  whole  body;  and  other  experiments  show  that 
the  great  vaso-motor  centre  of  all  the  vessels  is  situated  in 
the  neighbourhood  of  the  medulla  oblongata;  and  that  while 
the  vaso-motor  nerves  of  the  head  and  neck  leave  the  cord 
at  the  base  of  the  neck  to  reascend  in  the  sympathetic,  those 
to  the  rest  of  the  body  issue  by  the  anterior  roots  of  the 
spinal  nerves. 

The  heart  receives  its  nervous  supply  partly  from  the 
sympathetic,  and  partly  from  the  piieumogastric  nerve. 
Irritation  of  the  sympathetic  accelerates  its  action,  as  also 
does  irritation  of  the  branch  of  communication  between  the 
spinal  cord  and  the  inferior  cervical  ganglion  of  the  sym- 
pathetic. Thus  the  heart  may  be  said  to  receive  its 
motor  supply  similarly  to  the  arteries.  But  what  is  much 
more  remarkable  is,  that  irritation  of  the  pneumogastric 
diminishes  the  frequency  of  the  heart's  action,  and  if 
carried  sufficiently  far  arrests  it.  This  it  does  by  pre- 
venting in  some  way  the  normal  impulse  to  contraction; 
for  the  arrested  heart  has  its  walls  relaxed,  and  contracts 
on  application  of  direct  stimulus,  and  therefore  the  in- 
hibitory action,  as  it  is  called,  cannot  be  explained  either 
by  spasm  or  exhaustion. 

There  are  other  instances  of  similar  inhibitory  actions. 
Thus,  if  one  of  the  sensory  nerves  of  a  rabbit's  ear  be 
divided,  and  its  central  end  stimulated,  the  vessels  of  the 
ear  dilate;  and  in  the  case  of  the  submaxillary  salivary 
gland  of  a  rabbit,  while  irritation  of  the  sympathetic  con- 
tracts the  blood-vessels,  irritation  of  another  nerve  dilates 
them  and  increases  the  secretion.  Such  phenomena  have 
led  to  the  use  of  the  expression  inhibitory  nerves ;  but  the 
use  of  that  phrase  must  not  lead  it  to  be  supposed  that  the 
inhibitory  action  is  in  any  case  direct.  The  dilatation  of  the 
vessels  of  the  submaxillary  gland  is  explained  by  the  consider- 
ation that  nerves  end  in  the  secreting  cells,  and  that  increased 
action  of  the  cells  may  well  attract  more  blood;  and  cases 
which  cannot  be  explained  in  a  similar  fashion  may  possibly 
be  all  of  them  the  result  of  action  on  nervous  centres,  and 
not  on  terminal  organs.  It  is  not  conceivable  that  nervous 
impression  could  have  two  antagonistic  effects  on  a  muscle, 


THE   SYMPATHETIC   SYSTEM. 


according  as  it  came  by  one  nerve  fibre  or  another;  but  the 
heart  has  ganglia  within  it,  and  it  can  be  understood  that  a 
nervous  current  from  one  source  might  divert  another  current 
out  of  its  usual  channel,  or  possibly  oppose  its  passage  through 
a  ganglion. 


CHAPTER  XY, 

THE  SENSES. 

THE  senses  are  five  in  number,  namely,  common  sensation 
and  the  four  special  senses;  and  the  special  senses  are  natiir- 
ally  arranged  in  two  pairs,  taste  and  smell  giving  sensations 
of  a  simple  kind,  while  the  sensations  of  sight  and  hearing 
are  of  a  far  more  complex  description,  and  produced  by  the 
action  of  exceedingly  complex  organs. 

159.  Common  Sensation  includes  feeling  and  touch.  In 
feeling,  the  mind  has  simply  the  idea  of  a  condition  of  the 
body;  while  from  touch  it  receives  an  idea  of  properties  of 
external  objects.  Pain,  tickling,  and  a  sense  of  warmth  or 
cold  are  instances  of  feeling  which  are  not  necessarily  accom- 
panied with  touch. 

Although  the  variety  of  common  sensation  constituting  touch 
is  confined  to  the  surface  of  the  body  and  to  the  mouth,  and  the 
skin,  with  its  nerve  terminations,  already  described  (p.  68),  is 
the  principal  organ  of  this  sense,  feeling  is  not  confined  to  the 
surface.  Pain,  as  we  all  know,  may  be  felt  in  the  deep 
parts;  and  there  is  another  form  of  feeling  in  deep  tissues 
which  has  excited  a  good  deal  of  attention,  namely,  muscular 
sense.  Although  the  mind  is  unconscious  of  the  particular 
muscles  which  exist  in  any  part,  it  is  yet  able  to  regulate 
their  contraction,  so  as  to  produce  and  direct  every  move- 
ment with  precision,  which  it  could  not  do  without  a  know- 
ledge of  the  position  of  the  parts.  No  doubt  this  knowledge 
is  only  partly  due  to  a  sense  of  the  state  of  the  muscles,  as 
may  be  illustrated  by  the  consideration  that  in  moving  one's 
fingers  there  is  little  sense  of  action  save  in  the  fingers  them- 
selves, although  we  know  that  the  muscles  which  flex  and 
extend  the  fingers  are  really  in  the  forearm;  but  we  can 
make  the  muscles  of  a  limb  rigid  by  an  effort  of  the  will 


COMMON   SENSATION.  219 

without  changing  the  position  of  the  limb,  and  this  rigidity 
is  accompanied  with  a  peculiar  sensation.  Also,  the  sense  of 
muscular  exhaustion  or  weariness  is  derived,  in  part  at  least, 
from  the  muscles  themselves;  and  the  sense  of  resistance 
consists  partly  in  a  feeling  of  the  expenditure  of  a  certain 
amount  of  muscular  exertion  which  meets  with  opposition. 
That,  however,  is  only  one  element  in  the  sense  of  resistance, 
and  one  which  comes  into  play  only  when  there  is  muscular 
action;  but  suppose  that  the  head  is  reclined,  and  that 
something  comes  in  contact  with  it,  the  force  of  the  contact 
and  the  hardness  or  softness  of  the  object  are  both  appre- 
ciated, although  no  muscles  are  brought  into  play,  and 
although  there  are  none  in  the  part  which  has  been  touched. 
Obviously,  in  such  circumstances,  what  is  appreciated  is  the 
character  and  degree  of  the  pressure  exercised  against  the 
skin. 

And  now,  if  the  student,  having  considered  these  different 
varieties  of  feeling,  will  attempt  to  analyse  what  touch  con- 
sists in,  he  will  find  that  it  is  not  a  different  sense  from 
feeling,  but  only  an  application  of  it,  accompanied  with  a 
judgment  of  the  mind.  The  hardness  or  softness,  roughness 
or  smoothness,  of  the  object  touched,  exercise  different  kinds 
of  pressure  on  the  skin,  and  can  be  examined  by  means  of 
more  or  less  muscular  exertion;  while  the  sense  of  the  posi- 
tion of  the  touching  organ  enables  us  to  determine  the  form 
and  extent  of  the  object. 

The  sense  of  temperature  has  some  claim  to  be  considered 
as  distinct  from  all  others;  for  rare  instances  are  on  record 
of  the  loss  of  this  sense  in  a  limb,  leading  to  the  exposure 
to  severe  injury  from  burning,  while  the  other  varieties  of 
common  sensation  remained. 

160.  An  important  element  in  delicacy  of  touch  consists  in 
the  localization  of  the  feeling  excited  by  contact;  and  an  at- 
tempt has  been  made  to  measure  the  degree  of  sensitiveness  of 
different  parts  of  the  surface,  by  ascertaining  how  nearly  two 
points,  say  the  points  of  a  pair  of  compasses,  may  be  approached 
one  to  the  other,  and  yet  be  distinguished  as  separate  when 
simultaneously  laid  against  the  skin.  Judged  in  this  way, 
the  tongue  is  more  sensitive  than  the  fingers,  and  the  back 
has  very  little  sensation.  But  if  the  student  try  the  experi- 


220  ANIMAL   PHYSIOLOGY. 

ment  for  himself,  lie  will  find  that  this  is  a  test  simply  of  the 
power  of  localization  of  sensation,  and  that  of  the  three  parts 
mentioned,  the  back  is  the  most  acutely  affected  by  a  given 
amount  of  pressure;  it  is  the  part  on  which  the  pressure  of  a 
fine  point  most  quickly  causes  pain.  The  reason  of  this  is 
not  very  far  to  seek.  The  epidermis  of  the  back  is  much 
thinner  than  that  of  either  the  tongue  or  the  ^fingers,  and 
therefore  when  an  object  touches  it,  the  nerve-extremities 
are  less  protected.  On  the  other  hand,  the  nerve-extremities 
are  far  more  numerous  in  the  tongue  and  fingers,  and  a  given 
irritation  produces  a  slighter  impression  on  a  greater  number 
of  them.  When  the  impressed  condition  of  a  nerve  is  carried 
to  a  certain  pitch  of  intensity,  pain  is  the  result ;  but  when  a 
number  of  nerves  are  impressed  in  a  slighter  degree,  increased 
information  is  obtained.  Thus  it  happens  that  the  back, 
with  few  nerves  and  comparatively  thin  skin,  is  highly  sensi- 
tive, but  a  very  poor  tactile  organ;  while  the  fingers,  with 
many  nerves  and  thick  cuticle,  bear  rougher  usage  without 
pain,  at  the  same  time  that  they  are  useful  for  touch. 

A  very  common  experiment  illustrates  that  a  distinction 
must  be  drawn  between  the  localization  of  impressions  on  the 
surface  of  a  touching  organ,  and  the  sense  of  the  position  of 
the  organ,  spoken  of  in  a  previous  paragraph.  If  the  middle 
finger  be  crossed  over  the  back  of  the  forefinger,  and  a  pencil 
or  a  pea  be  placed  between  the  tips  of  the  two  fingers,  the 
sensation  produced  is  not  that  of  a  single  round  object,  but 
of  two  distinct  objects;  and  if  the  eyes  be  shut,  the  illusion 
will  be  complete.  The  reason  is,  that  while  each  finger  con- 
veys a  correct  tactile  sensation,  the  mind  fails  to  recognise 
the  exact  relative  positions  of  the  unusually  placed  tactile 
surfaces. 

161.  All  the  finer  differences  of  touch  disappear  in  pain. 
Heat,  cold,  chemical  and  mechanical  irritation,  all  produce 
pain  when  applied  in  excess;  and  although  the  character  of 
the  pain  varies  with  the  nature  of  the  damage  done  to  tex- 
ture, in  no  case  is  there  any  resemblance  between  the  pain 
and  the  sensation  caused  by  a  minor  degree  of  the  stimulus. 

Pain,  then,  may  be  considered  as  a  sensation  distinct  from 
others,  and  resulting  from  an  excess  of  the  impressed  condi- 
tion in  a  sensory  nerve;  indeed,  it  may  be  felt  in  parts  which 


SMELt.  221 

are  not  otherwise  endowed  with  feeling,  as,  for  example,  the 
stomach  and  intestines.  Allied  to  it  there  are  various  other 
sensations,  not  belonging  to  any  of  the  five  senses,  but  which 
may  be  here  alluded  to.  Among  them  are  some  which  are 
pathological,  such  as  numbness,  arising  from  derangement  of 
the  conditions  necessary  for  the  normal  activity  of  the  nerves 
of  any  part,  and  giddiness  and  nausea,  which  owe  their  origin 
to  deranged  conditions  within  the  brain.  Hunger  and  thirst 
are  more  healthy  sensations,  which  are  of  an  exceedingly 
curious  kind;  for  while  the  one  is  felt  in  the  stomach,  and 
the  other  in  the  throat,  both  are  greatly  dependent  on  the 
general  state  of  nutrition  of  the  body.  Indigestible  sub- 
stances give  only  a  very  temporary  relief  from  hunger,  while, 
on  the  other  hand,  the  stomach  may  be  quite  empty  without 
the  sensation  existing;  and  thirst  is  only  partially  relieved 
by  the  mere  contact  of  fluid  with  the  fauces. 

162.  Smell,  the  sense  by  which  we  distinguish  odours,  is 
located  within  the  nasal  cavities,  and  depends  on  a  simpler 
mechanism  than  any  other  of  the  special  senses. 

The  nasal  cavities  or  fossse  extend  from  the  nostrils  back 
to  the  pharynx,  into  which  they 
open  behind;  the  communication 
being  termed  the  posterior  nares 
(fig.  49).  Superiorly,  they  are 
separated  from  the  cranial  cavity 
by  a  thin  plate  of  bone,  the 
cribriform  plate  of  the  ethmoid, 
which  is  perforated  by  the  fila- 
ments of  the  olfactory  nerve ;  and 
inferiorly  their  floor  is  made  by 
the  hard  and  soft  palate;  while 
between  them  is  placed  a  vertical 
septum  dividing  one  fossa  or  Fig.  109.— NASAL  FOSSJB, 
cavity  from  the  other.  transverse  vertical  section. 

The  outer  wall,  in  the  human  subject,  presents  three  ledges 
of  bone,  one  above  another,  projecting  inwards  with  a  down- 
ward curve,  and  termed  turbinated  bones.  The  inferior  is  a 
distinct  bone;  while  the  others  are  portions  of  the  ethmoid, 
and  project  little  lower  than  the  floor  of  the  orbit. 

The  ethmoid  bone  consists  of  a  central  plate,  the  cribriform 


222  ANIMAL   PHYSIOLOGY. 

plate,  and  two  lateral  masses.  The  central  plate  descends  in 
the  middle  line  from  the  cribriform  plate,  and  forms  the 
upper  part  of  the  nasal  septum;  while  the  lower  part  of  the 
septum  is  formed  behind  by  another  bone,  the  vomer,  and  in 
front  is  cartilaginous.  The  lateral  masses  of  the  ethmoid 
form,  by  two  smooth  surfaces,  part  of  the  inner  walls  of  the 
orbit,  and  are  hollowed  out  into  air  cells,  with  thin  papery 
walls  lined  with  mucous  membrane,  and  communicating  with 
the  nasal  fossae.  The  ethmoidal  turbinated  bones  form  part 
of  these  lateral  masses.  The  superior  one  exists  only  in  the 
hinder  half  of  each  mass,  and  overhangs  a  space  between  it 
and  the  middle  turbinated  bone  called  the  superior  meatus  of 
the  nose,  which  communicates  in  front  with  ethmoidal  cells, 
and  has  the  opening  of  a  space  hollowed  out  of  the  sphenoid 
bone,  the  sphenoidal  sinus,  opposite  it  behind  (fig.  17).  The 
other  turbinated  bone  of  the  ethmoid,  called  £he  middle 
turbinated  bone,  is  at  the  lower  part  of  the  lateral  mass,  and 
overhangs  a  gallery  between  it  and  the  inferior  turbinated 
bone,  the  middle  meatus  of  the  nose.  This  communicates 
with  a  hollow  in  the  upper  jaw,  the  maxillary  sinus  or 
antrum,  and  also,  by  a  passage  through  the  fore  part  of  the 
lateral  mass  of  the  ethmoid,  with  the  frontal  sinus,  a  large 
hollow  in  the  frontal  bone,  in  the  lower  part  of  the  forehead. 
A  third  gallery  or  passage,  the  inferior  meatus,  lies  between 
the  inferior  turbinated  bone  and  the  floor  of  the  nose;  and 
into  it  there  opens  near  the  front  the  nasal  duct,  a  canal  by 
which  the  tears  are  conveyed  from  the  eye;  while,  opposite 
its  extremity  behind,  is  the  orifice  of  a  communication  lead- 
ing from  the  ear,  the  Eustachian  tube. 

163.  I  have  thought  it  better  to  give  the  student  at  once  a 
connected  account  of  the  whole  interior  of  the  nose,  but  it 
must  not  be  supposed  that  all  the  structures  now  described 
are  connected  with  smell.  What  is  called  the"  nose,  in  the 
more  extended  sense  of  the  word,  has  a  number  of  different 
functions.  The  nose  proper,  or  feature  so  called,  is  simply 
an  organ  of  expression.  The  nostrils  are  of  use  as  the  open- 
ings into  the  nasal  cavities,  but  the  prominence  above  them 
is  merely  ornamental.  The  nasal  cavities  are  connected 
with  three  functions — breathing,  voice,  and  smell.  Only 
the  ethmoidal  part  has  the  filaments  of  the  olfactory  nerve 


SMELL.  223 

distributed  in  its  mucous  membrane;  the  lower  part  is 
furnished  with  ciliated  epithelium  like  the  rest  of  the  respir- 
atory tract,  as  is  also  the  part  of  the  pharynx  into  which  it 
opens;  and  in  quiet  breathing  the  upper  edge  of  the  epiglottis 
comes  almost  in  contact  with  the  edge  of  the  soft  palate 
which  forms  its  floor  behind.  When  the  mouth  is  shut,  the 
air  passes  to  and  from  the  larynx  through  the  nasal  fossae; 
and,  even  when  the  mouth  is  open,  a  considerable  quantity 
of.  air  passes  through  them;  and  the  current  is  broken  by 
the  various  turbinated  projections,  so  that  part  of  it  is 
directed  upwards  to  the  olfactory  region,  and  the  whole  of  it 
is  assimilated  to  the  temperature  of  the  body  before  entering 
the  larynx.  In  "  sniffing,"  or  drawing  air  into  the  nose  to 
assist  smell,  the  inspirations  are  short,  abrupt,  and  repeated, 
so  as  to  put  small  quantities  of  air  into  rapid  motion;  and 
in  each  inspiration  there  is  a  slight  but  quick  contraction  of 
the  nostrils,  both  increasing  the  rapidity  of  the  air  and 
directing  it  upwards. 

Both  the  nasal  fossae  and  the  various  sinuses  opening  off 
from  them  act  as  reverberating  cavities  to  improve  the 
timbre  of  the  voice;  and  it  is  in  connection  with  the  voice, 
not  with  smell,  that  the  nasal  fossae  in  the  human  subject 
are  extensive,  They  differ  from  those  of  quadrupeds  in  their 
greater  vertical  height  and  diminished  extent  from  before  back- 
wards, and  in  communicating  with  larger  and  more  nume- 
rous sinuses.  But,  in  addition,  it  will  be  seen  on  comparison 
with  any  common  mammal,  such  as  the  dog  or  the  sheep, 
that  in  the  human  subject  there  is  a  very  small  amount  of 
surface  provided  for  the  distribution  of  the  olfactory  nerves. 
In  those  animals  the  turbinated  bones  are  far  more  complex, 
and  finer  secondary  turbinations  come  off  from  the  main 
laminae,  while  all  are  directed  with  their  ends  towards  the 
nostrils,  so  that  the  inhaled  air  is  exposed  to  a  most  exten- 
sive sensitive  surface.  This  corresponds  with  what  one 
would  expect,  both  from  the  very  small  comparative  size  in 
man  of  the  olfactory  bulbs  of  the  brain,  and  from  the  ob- 
viously acute  sense  of  smell  in  the  lower  animals,  exceeding 
in  some  of  them,  such  as  the  dog,  anything  which  our  own 
senses  enable  us  to  conceive. 

164.  The  olfactory  nerves  consist  each  of  a  brush  of  fila- 


224: 


ANIMAL   PHYSIOLOGY. 


B 


merits  of  the  soft  and  nucleated  variety,  and  ramify  beneath 
the  mucous  membrane  of  the  upper  and  middle  turbinated 
bones  and  the  ethmoidal  part  of 
the  nasal  septum;  and  in  this, 
which  is  termed  the  olfactory 
region,  the  mucous  membrane  of 
the  nose,  called  also  the  Schnei- 
derian  or  pituitary  membrane,  is 
softer  and  smoother,  and  has  the 
mucous  glands  smaller  than  they 
are  in  the  lower  part  of  the  nasal 
fossse.  The  olfactory  mucous  mem- 
brane  is  likewise  distinguished  by 
being  clothed  with  a  non-ciliated 
columnar  epithelium.  But  be- 
tween the  ordinary  columnar  cells 
are  scattered  slender  nucleated 
bodies,  each  of  which  is  in  con- 
tinuity with  a  filament  of  olfac- 
tory nerve,  and  in  birds  and 
amphibia  is  furnished  with  a 
single  hair  or  a  bundle  of  fine 
cilia.  These  are  called  olfactory 
Fig.  110. —OLFACTORY CELLS,  cells;  and  we  are  led  to  believe 
with  the  epithelial  cells  be-  that  tlie  WOnderfully  and  impon- 
tween  which  they  lie;  both  -,  ,  ,  .  -,  J 

with  deep  connections.    A,   clerably  minute  odorous  particles 
Human.      B,    From   frog,   drawn   into   the    nasal  fosssa    in 
After  Schultze  and  Frey.       inspiration  affect    their  extremi- 
ties, and,  through  them,  the  olfactory  nerves. 

185.  Taste  is  a  sense  which  is  closely  allied  to  both 
common  sensation  and  smell,  and  as  it  is  less  definite  in  its 
nature  than  the  other  special  senses,  so  also  it  is  dependent 
on  a  less  definitely  distinguished  nervous  supply;  for  while 
smell,  sight,  and  hearing,  have  each  a  special  nerve  devoted 
to  them,  the  organ  of  taste  has  its  sensory  supply  from  two 
mixed  nerves,  the  glosso-pharyngeal  and  the  gustatory  or 
lingual  branch  of  the  fifth. 

The  organ  of  taste  is  the  upper  surface  of  the  tongue. 
This  is  covered  with  papillce,  which  differ  from  those  on  the 
general  surface  of  the  skin,  not  only  in  being  larger,  but  also 


TASTE. 


225 


in  being  compound.    The  papillae  are  of  three  kinds.    Thickly 

disposed  over  the  whole  surface  are  those  of  smallest  size, 

the  filiform  kind,  which  are  long  and 

slender    prominences,    as    their   name  j 

indicates,    and    have     a    few    simple/ 

papillae  at  their  extremities.    Scattered  ' 

sparsely   among  the   filiform   are   the 

fungiform    papillae,     which    have     a 

rounded  shape,  and  sometimes  remain 

red   when  the  rest  of  the  tongue   is 

furred,  giving  a  spotted  appearance  to 

the  surface.     The  papillae  of  the  third 

description    are    called    circumvallate, 

and  are  several  times  larger  than  the 

fungiform.     They  are  usually  less  than 

a  dozen  in  number,  and  are  arranged 

near  the  back  of  the  tongue    in  two 

lines  diverging  from  the  mesial  groove  Fig.      111.  —  TONGUE, 

in  a  curved  fashion.     They,  as  well  as      showing  the ,  circum- 

the  fungiform  kind,  have  simple  papillae      £^^4  "££ 

on  their  broad  tops,  and  they  get  their      fungiform     scattered 

name  from    each    being  sunk    into  a      over  the  surface. 

hollow,  whose  outer  wall  rises  up  like  a  rampart  to  a  level 

with  it. 

The  epithelium  which  covers  the  tongue  is  of  a  thick 
stratified  squamous  description ;  and  the  variations  in  the 
distinctness  of  the  papillae,  in  different  states  of  health,  depend 
more  on  the  condition  of  the  epithelium  than  on  differences 
in  the  prominences  which  they  clothe. 

166.  The  sensory  nerves  of  the  tongue  end  in  a  variety  of 
ways.  End-bulbs  (p.  68)  are  found  in  the  papillae  close  to 
their  extremities.  Also,  on  the  frog's  tongue,  nerves  have  been 
traced  into  elongated  cells  with  forked  extremities,  situated 
in  the  epithelium;  and  although  the  mammalian  tongue  is 
very  different  from  that  of  the  frog,  yet  it  is  likely  that  some 
similar  mode  of  nerve-termination  is  scattered  over  the 
tongue  of  the  mammal  also;  for  forked  cells,  continuous 
with  nerve-fibres,  have  been  described  in  the  epidermis  of 
general  integument.  But  the  most  remarkable  mode  of 
nerve-termination  found  in  the  tongue  is  in  the  taste-cones  of 
H  p 


226 


ANIMAL   PHYSIOLOGY, 


Schwalbe  and  Loven.  These  are  structures  -which  are  found 
nowhere  else  but  on  the  protected  sides  of  the  circumvallats 
papillae.  Each  cone  occupies  the  whole  thickness  of  the 
epithelium;  its  base  is  on  the  papilla,  its  apex  at  the  sur- 
face, and  its  sides  are  convex.  It  consists  of  vertically 
placed  nucleated  cells  running  the  whole  length  of  the  cone, 
the  outer  of  which  are  flattened  like  the  staves  of  a  barrel, 
and  form  coats  like  those  of  an  onion,  while  the  inner  are 
rod-shaped,  and  some  of  them  with  hair-like  extremities,  like 
the  olfactory  cells;  and  the  group  of  these  extremities  at  the 
top  of  each  cone  projects  into  the  fossa  round  the  papilla, 
through  a  small  opening  left  between  the  flattened  superficial 
cells  of  the  surrounding  epithelium. 


Fig.  112. — TASTE-CONES  OF  SHEEP.  A,  Vertical  section  of  circum- 
vallate  papilla,  exhibiting  seven  taste-cones  on  each  side.  B, 
Outer  cells  of  taste-cone.  C,  Inner  cells.  Schwalbe. 

167.  All  tastes  are  not  perceived  by  the  same  means.  Astrin- 
gents are  perceived  when  applied  to  the  fore  part  of  the 
tongue,  but  not  when  applied  to  the  back  part.  Bitters  are 
perceived  when  applied  to  the  back  part,  but  do  not  affect 
the  fore  part.  Sweet  and  saline  tastes  are  perceived  by 
means  of  both  the  fore  and  back  part,  but  most  acutely  by 
the  fore  part.  If  sugar  be  laid  on  the  tip  of  the  tongue,  the 
sweetness  is  at  once  perceived,  though  not  so  acutely  as  after 
pressing  the  tongue  to  the  palate;  but  it  may  be  rubbed  into 
the  back  of  the  tongue,  as  also  into  the  palate,  without  any 


VISION*.  227 

taste  being  detected  till,  the  palate  and  that  part  of  the  tongue 
are  pressed  together  in  swallowing.  Perhaps  the  accurate 
pressure  of  the  palate  is  necessary  to  bring  the  dissolved 
sugar  into  contact  with  the  taste-cones. 

In  what  is  termed  flavour,  there  is  something  more  than 
mere  taste.  The  texture  of  the  substance  tasted  is  an  ele- 
ment which  enters  into  flavour;  for  example,  the  smoothness, 
roughness,  hardness,  or  softness ;  and  these  are  appreciated  by 
the  acute  common  sensation  in  the  tongue  and  palate.  Smell 
is  another  element  in  flavour ;  and,  besides  that,  many  tastes 
and  smells  are  so  associated,  that  the  smell  brings  the  taste 
to  mind,  and  certain  aromatic  tastes  even  suggest  what  seem 
in  some  way  correspondent  odours  to  the  imagination.  It  is  a 
matter  of  common  observation,  that  interference  with  smell, 
as  during  an  attack  of  catarrh,  interferes  with  the  power  of 
distinguishing  flavours. 

There  is  another  curious  circumstance  which  points  to  a 
connection  between  taste  and  smell.  In  a  number  of  mam- 
mals there  is  a  sensory  organ  on  each  side  of  the  septum  of 
the  nose,  close  to  the  floor,  called  the  organ  of  Jacobson.  It 
is  a  pouch  lined  with  mucous  membrane,  receiving  a  twig 
from  the  olfactory  nerve,  and  another  from  the  source  which 
supplies  common  sensation  to  the  nose  and  palate;  and  its 
orifice  is  pointed  downwards,  so  that  in  many  animals  it  is 
most  easily  entered  through  a  canal  in  the  fore  part  of  the 
palate.  The  use  of  this  organ  is  hard  to  conceive;  but  it 
brings  the  distribution  of  the  olfactory  nerve  very  close  to 
the  organ  of  taste. 

The  back  part  of  the  tongue  is  supplied  by  the  glosso- 
pharyngeal  nerve,  but  the  anterior  three-fourths  almost 
entirely  by  the  gustatory  branch  of  the  fifth;  and  it  is  dim- 
cult  to  avoid  the  conclusion  that  both  nerves  are  really  nerves 
of  taste. 

168.  Vision. — If  by  means  of  the  eye  we  merely  had  a 
sensation  varying  in  intensity  according  to  the  amount,  and 
in  character  according  to  the  colours  of  the  light  before  us, 
vision  would  be  a  sense  completely  comparable  with  taste  or 
smell.  But  to  produce  the  effect  of  the  landscape,  there  is 
required,  in  addition,  an  exceedingly  fine  power  of  localize,' 
tion  of  the  impressions  made  by  different  rays,  not  indeed  a 


228  ANIMAL    PHYSIOLOGY. 

power  which  enables  us  to  perceive,  as  in  touch,  the  spot 
where  the  stimulus  is  applied,  but  one  by  which  the  relative 
positions  of  all  the  rays  cantering  the  eye  at  one  time  may  be 
recognised. 

It  is  further  necessary  that  every  nerve-termination  or 
sensitive  point  in  the  eye  shall  receive  only  one  ray  at  a  time, 
and  that  it  shall  be  the  proper  ray.  In  the  case  of  insects, 
this  is  managed  by  every  nerve-termination  being  placed  at 
the  bottom  of  a  long  dark- walled  tube,  so  that  it  is  affected 
by  none  but  the  ray  which  falls  vertically  on  it.  But  in  all 
vertebrata,  as  well  as  in  the  cephalopodous  molluscs,  of  which 
the  cuttle  fishes  are  a  familiar  example,  the  object  is  achieved 
in  much  greater  perfection  by  an  optical  apparatus  which 
throws  the  inverted  image  of  the  landscape  on  a  sensitive 
surface  at  the  bottom  of  a  dark  chamber. 

The  whole  optical  apparatus,  as  well  as  the  sensitive  sur- 
face, is  contained  within  the  eyeball ;  the  range  of  vision  is 
increased,  and  the  two  eyes  are  enabled  to  act  in  concert,  by 
means  of  muscles  which  turn  the  eyeballs;  and  inasmuch 
as  the  fore  part  of  the  eye  must  be  preserved  from  opacity, 
whether  from  dryness,  scratching,  or  the  nutritive  changes 
consequent  on  irritation,  it  is  protected  by  the  eyebrows, 
eyelids,  eyelashes,  and  a  lachrymal  apparatus. 

169.  The  eyeball  is  a  nearly  spherical  structure,  about  an 
inch  in  diameter,  pierced  at  the  back,  at  a  point  about  a  tenth 
of  an  inch  internal  to  the  centre,  by  the  optic  nerve,  which, 
being  in  its  sheath  a  stout  cylinder  a  sixth  of  an  inch  thick, 
looks  like  the  stalk  of  a  berry.  The  outer  investment  of  the 
eyeball  is  protective,  and,  in  the  greater  part  of  its  extent,  is 
an  exceedingly  tough  felted  fibrous  coat,  called  the  sclerotic, 
thickest  behind;  but  in  front  it  becomes  abruptly  trans- 
parent, so  as  to  form  a  clear  window,  the  cornea,  through 
which  one  can  look  into  the  interior.  At  the  edge  of  junc- 
tion, the  fibres  of  the  sclerotic  are  continuous  with  those  of 
the  cornea,  the  same  bundle  being  opaque  in  the  outer  part 
of  its  extent,  and  transparent  in  the  inner;  but  in  the  cornea 
the  fibres  are  arranged  in  numerous  parallel  laminae,  with 
intercommunicating  branched  spaces  between  them.  The 
cornea  is  altogether  destitute  of  blood-vessels,  though  sup- 
plied with  a  network  of  nerves  near  its  surface;  and  it  is 


VISION.  229 

limited  behind  by  an  elastic  lamina,  and  covered  in  front 
with  epithelium.  The  lining  membrane  of  the  eyelids  and 
front  of  the  eye  is  called  the  conjunctiva ;  it  is  continuous 
with  the  skin  at  the  margin  of  the  lids;  inside  the  lids  it  is 
pink  with  blood-vessels;  where  reflected  on  the  sclerotic  or 
white  of  the  eye,  it  becomes  more  delicate  and  transparent, 
although  still  a  distinct  membrane  easily  detached;  but  when 
ifc  reaches  the  cornea,  every  structure  corresponding  with 
the  cutis  vera  is  lost,  and  there  remains  only  a  stratified 
epithelium  closely  adherent  to  the  proper  corneal  structure. 
No  blood-vessels  lie  beneath  this  epithelium  to  interfere  with 
vision;  but  when  inflammation  is  excited,  vessels  push  their 
way  inwards  with  a  rapidity  which  is  exceedingly  interest- 
ing, as  showing  how  speedily  capillaries  can  be  developed.* 


Fig.  113. — HUMAN  EYE  enlarged,  with  part  of  the  cornea  and  scle- 
rotic removed,  a-,  Sclerotic;  6,  cornea;  c,  choroid  coat,  showing 
arteries  and  veins,  and  the  ciliary  nerves  on  the  surface ;  d,  iris ; 
e,  pupil;  /,  ciliary  muscle.  , 

170.  Looking  at  the  living  eye,  one  sees,  through  the  trans- 
parent cornea,  the  part  which  is  coloured.  This  is  a  muscular 
curtain,  the  iris,  with  a  circular  perforation  in  the  centre,  the 

*  An  injection  by  Stirling,  in  my  possession,  beautifully  demon- 
strates that  the  conjunctival  network  of  capillaries,  on  reaching  the 
edge  of  the  cornea,  is  reflected  and  continuous  with  a  deeper  network 
belonging  to  the  sclerotic.  A  similarly  reflected  sheet  of  capillaries 
is  described  by  Hyrtl  at  the  attachment  of  the  round  ligament  of  the 
hip-joint  to  the  femur. 


230 


ANIMAL    PHYSIOLOGY. 


pupil,  which,  appears  black,  because  few  of  the  rays  of  light 
which  enter  it  are  reflected  from  the  camera  behind.  The 
iris  is  attached  round  about  to  the  margin  of  the  sclerotic, 
where  it  meets  the  cornea;  and  it  has  two  sets  of  muscular 
fibres;  a  circular  set  round  its  pupilary  margin,  which  con- 
tracts the  pupil;  and  a  radiating  set  towards  its  circumference, 
by  which  the  pupil  is  dilated.  The  circular  fibres  are 
governed  by  the  third  cranial  nerve,  the  radiating  fibres  by 
the  sympathetic.  Division  of  the  sympathetic  in  the  neck 
causes  the  pupil  to  contract,  while  stimulation  of  the  divided 
trunk  causes  it  to  dilate. 


A 


Fig.  114. — PIGMENT  CORPUSCLES  Fig.  115. — SECTION  OF  CHOROID 

OF   THE   CHOROID    COAT.      A,  OF     THE     Ox,     diagrammatic 

Branched  corpuscles  from  the  view,     a,  Arteries  and  veins; 

connective  tissue  of  the  choroid  b,  arterioles  and  venous  radicles 

coat :  the  white  spot  in  each  is  piercing  the  tapetuin ;  c,  mein- 

the  nucleus.      B,  Pigmentary  brane  of  Ruysch. 
epithelium  on  the  inner  surface 

.    of  the  choroid  coat. 

In  making  a  dissection  of  the  eye,  such  as  every  student 
may  easily  make  for  himself  on  the  eye  o'f  a  sheep  or  an  ox, 
if  the  sclerotic  and  cornea  be  carefully  removed,  there  is 
brought  into  view  a  second  coat,  the  tunica  vasculosa,  of  which 
the  iris  is  the  anterior  part,  while  the  posterior  part,  cor- 
responding in  extent  with  the  sclerotic,  is  called  the  choroid. 
The  choroid  coat  consists  of  exceedingly  closely-set  small 
arteries  and  veins,  imbedded  in  connective  tissue,  the  branched 
corpuscles  of  which  are  loaded  with  pigment;  and  the  capil- 
laries uniting  these  vessels  are  thrown  inwards  to  the  deep 
aspect  of  them,  where  they  form  one  of  the  closest  networks 
in  the  body,  the  membrane  of  Ruysvh.  Behind  the  periphery 


VISION.  23l 

of  the  iris,  tlie  blood-vessels  of  the  choroid  form  a  corona  of 
richly  vascular  projections  directed  inwards,  and  capable  of 
considerable  variation  in  size  according  to  their  degree  of 
gorgement:  these  are  called  the  ciliary  processes;  and  con- 
nected with  their  roots  is  the  ring  of  tissue  which  fastens  the 
iris  to  the  sclerotic,  a  white  ring  consisting  principally  of 
muscular  fibres,  the  ciliary  muscle. 

The  whole  of  the  deep  surface  of  the  choroid  and  back  of 
the  iris  is  lined  with  epithelial  cells  loaded  with  dark  brown 
pigment,  which  opposite  the  choroid  are  flat,  hexagonal,  and 
arranged  in  a  single  layer,  but  behind  the  iris  are  more 
densely  accumulated.  In  persons  with  brown  eyes,  the 
colour  of  the  iris  is  due  to  a  deposit  of  pigment  between  its 
anterior  fibres,  corresponding  with  the  branched  pigment- 
corpuscles  between  the  vessels  of  the  choroid;  but  in  those 
with  blue  eyes,  the  iris  is  devoid  of  pigment,  and  the  colour 
is  due  to  the  effect  of  the  dark  pigment  behind  its  white 
substance.  In  Albinos,  persons  in  whom  there  is  an  absence 
of  brown  pigment  from  all  the  situations  in  which  it  is  com- 
monly found,  the  white  hair  and  eye-lashes  are  accompanied 
with  pink  eyes,  the  colour  of  which  is  due  to  the  blood, 
lighted  up,  not  only  with  reflected,  but  also  transmitted  light. 

The  pigment  of  the  choroid  prevents  reflection  of  rays  from 
the  parts  of  the  interior  of  the  eye  on  which  they  fall,  to  other 
points  of  the  sensitive  surface,  which,  were  it  to  occur,  would 
blur  the  image  of  the  landscape,  as  is  illustrated  by  the 
imperfect  vision  from  which  albinos  suffer.  But,  if  the 
student  choose  for  dissection  the  eye  of  a  sheep  or  some 
other  domestic  animal,  he  will  find  on  the  inner  surface  of 
the  choroid  a  state  of  matters  not  existing  in  man,  namely,  a 
portion,  opposite  and  above  the  pupilary  aperture,  which  has 
an  intense  satin-like  whiteness.  This  is  called  the  tapetum, 
and  by  reflecting  the  rays  to  the  part  of  the  sensitive  surface 
immediately  in  contact  with  it,  is  of  service  in  utilising  a  dim 
light,  and  enabling  these  animals  to  see  at  night.  It  is  this 
tapetum  seen  through  the  humours,  which  gives  the  brilliant 
greenish  light  to  the  pupil  of  the  cat. 

171.  In  the  course  of  dissection,  let  the  choroid  coat  be  gently 
torn  open  and  raised  from  the  subjacent  structures.  If  this 
be  done  with  due  care,  there  will  be  seen  laid  over  a  globe  of 


232  AtflMAL  PHYSIOLOGY. 

transparent  substance,  a  soft,  whitish,  pulpy  membrane,  the 
retina,  or  internal  tunic  of  the  eyeball.     The  retina  is  a 

nervous  structure,  containing 
the  distribution  of  the  optic 
nerve,  a  layer  of  nerve-corpus- 
cles, and  the  nerve-terminations 
on  which  the  rays  of  light  act; 
it  is  adherent  to  the  other  tunics 
at  the  optic  pore,  the  place  where 
the  optic  nerve  pierces;  and  it 
conies  to  an  apparent  margin 
in  front,  not  far  from  the  ciliary 
processes.  This  margin,  in  the 
human  eye,  is  scalloped,  and 
called  the  ora  serrata.  In  the 
human  eye  also,  when  a  per- 
Fig.  116.— SECTION  OP  HUMAN  fectly  fresh  specimen  is  ex- 
Em  a.  Sclerotic;  b,  cornea;  amilied  the  retina  will  be  seen 
c,  conumctiva ;  a,  iris :  c,  ,  -,  -,  -. 

crystalline  lens  closely  in-  **>  be  nearl7  transparent,  and 
vested  with  its  capsule,  and,  of  a  delicate  pmk  tint;  and 
above  and  below,  sections  of  there  will  be  noticed,  directly 
the  canal  of  Petit,  bounded  opposite  the  centre  of  the  pupil, 
in  front  by  suspensory  liga-  .-i  .  •  .  j.  ±1  * 

ment,  and  behind  by  hyalSid  *M  1S  ^  say,  a  tenth  of  an 
membrane.  The  radiating  inch  outside  the  optic  pore,  a 
white  lines  round  the  lens  structure  which  does  not  exist 
are  the  most  prominent  parts  in  domestic  animals,  namely,  the 
of  the  ciliary  processes,  the  77  .  /•  «..  •  rf 

only  parts  uncovered  with  2/f/T  f°*  °/  Sfymmnff,  an 
dark  pigment  ;  /,  ciliary  elliptical  mark,  of  a  yellow 
muscle  in  section ;  g,  retina,  colour,  with  a  depression  in 
with  ora  serrata  in  front ;  h,  the  middle,  called  fovea  cm- 
optic  nerve.  .  -i  • , 

The  retina,  notwithstanding  its  being  so  thin,  is  one  of 
the  most  complex  structures  in  the  body,  and  reveals  this 
complexity  when,  after  suitable  preparation,  it'  is  examined 
under  the  microscope  in  sections  made  vertically  through,  it. 
Close  to  the  surface  which  rests  on  the  transparent  media 
of  the  eye,  is  a  layer  of  fine  nerve  fibres,  the  expansion 
of  the  optic  nerve,  and  beneath  this  a  layer  of  multipolar 
nerve-corpuscles;  and  in  these  strata  are  the  ramifications  of 
the  retinal  artery,  which  breaks  UD  into  branches  at  the 


VISION. 


233 


117. — POSTERIOR  HALF 
OF  EYEBALL,  exhibiting  the 
retinal  vessels  ramifying 
from  the  optic  pore,  and, 
in  the  centre,  the  macula 
lutea  of  Sommering  with 
its  fovea  ceiitralis. 


optic  pore,  and  is  there  placed  on  the  surface  of  the  retina. 
Subjacent  to  the  multipolar  corpuscles  are  other  layers,  marked 
by  the  presence  of  nuclear  ele- 
ments; and  on  the  other  side  of 
these,  resting  on  the  choroidal 
epithelium,  is  what  is  termed  the 
bacillary  layer  (fig.  118). 

The  bacillary  layer,  or  Jacob's 
membrane,  consists  of  multitudes 
of  minute  structures,  called  rods 
and  cones,  placed  vertically  to  the 
rest  of  the  retina,  like  the  ele- 
ments of  a  columnar  epithelium. 
The  rods  are  the  more  numerous,  Fig. 
and  consist  of  an  outer  and  inner 
part  of  dissimilar  nature :  the 
cones  have  an  outer  part  similar 
to  the  rods,  while  the  inner  part 
is  swollen  to  a  flask-shape,  and 
they  are  more  distinctly  connected,  through  the  medium  of 
structure  in  the  nuclear  layers,  with  the  multipolar  cor- 
puscles. In  the  yellow  spot,  only  cones  are  present  in  the 
bacillary  layer,  and  these  are  crowded  together,  and  of  smaller 
size  than  elsewhere;  also  the  multipolar  corpuscles  are 
numerous,  and  there  are  no  fibres  of  the  optic  nerve. 

172.  When  the  choroidcoat  has  been  divided,  the  transparent 
structures  which  occupy  the  cavity  of  the  eyeball  can  be 
removed  in  one  mass.  They  adhere  most  closely  to  the 
ciliary  processes,  but,  when  separated  from  them,  they  have 
the  appearance  of  a  limpid  globe  of  delicate  jelly,  in  the  fore 
part  of  which  is  placed  a  bead  of  denser  consistence,  sur- 
rounded by  a  plicated  collar,  whose  plications  fit  in  between 
the  ciliary  processes.  The  main  mass  is  called  the  vitreous 
humour;  it  consists  of  water  entangled  in  meshes  of 
transparent  tissue,  and  is  limited  by  a  structure  of  firmer 
consistence,  called  from  its  limpidity  the  hyaloid  membrane. 
The  denser  bead  in  front  is  the  crystalline  lens;  and  if  a 
score  be  made  along  the  face  of  it  with  a  needle  or  a  point  of 
a  knife,  the  capsule  which  retains  it  in  its  place  will  be  rup- 
tured, and  the  lens  will  start  out.  It  is  expelled  usually  with 


234 


ANIMAL   PHYSIOLOGY. 


a  distinct  degree  of  force, 
because  the  anterior  wall 
of  its  capsule  is  strong 
and  elastic:  the  posterior 
wall  is  fused  with  the  hya- 
loid membrane.  The  pli- 
cated collar  outside  the 
capsule  of  the  lens  is  called 
the  zonule  of  Zinn;  and  if 
a  small  tube  be  gently 
pushed  into  it  near  to  its 
inner  edge,  and  air  be 
blown  in,  there  will  be 
seen  to  be  a  cavity  sur- 
rounding the  periphery  of 
the  capsule,  the  anterior 
wall  of  which  is  formed  by 
a  plicated  fibrous  mem- 
brane distinct  from  the 
hyaloid.  This  cavity  is 
called  canal  of  Petit;  and 
f  xthe  membrane  in  front  of 
i>*  it,  extending  from  the 
most  prominent  part  of  the 
hyaloid  membrane  to  the 
anterior  wall  of  the  capsule 
of  the  lens,  is  named  the 
H*.: (^/suspensory  ligament. 

The  crystalline  lens  is 
about  a  third  of  an  inch  in 
diameter,  and  a  fifth  from 
front  to  back,  and  is  more 
convex  behind  than  in 
front.  It  consists  of  layers 

Fig.  118. — RETINA,  diagrammatic  view  of  the  structures  seen  in 
vertical  section.  A,  General  view.  B,  The  nervous  elements. 
a,  Bacillary  layer;  6,  membraiia  limitans  externa ;  c,  external 
nuclear  layer ;  d,  external  granular  layer ;  e,  internal  nuclear 
layer  ;  /,  internal  granular  layer;  g,  ganglionic  layer  ;  h, 
branches  of  optic  nerve ;  f,  membrana  limitans  interna.  After 
Schultze. 


VISIOK.  235 

of  substance,  one  within  another,  like  an  onion;  and  the 
layers  increase  in  density  towards  the  centre,  which  is  so 
firm  that  it  is  sometimes  called  the  nucleus  of  the  lens. 
The  layers  are  composed  of  fibres  extending  from  front  to 
back,  each  with  a  nucleus,  and  remarkable  in  having  serrated 
edges  by  which  they  fit  into  one  another. 

The  capsule  of  the  lens  is  in  contact  in  front  with  the 
inner  edge  of  the  iris,  and  there  is  a  space  left  between  it 
and  the  cornea.  This  is  filled  with  fluid,  the  aqueous 
humour ;  and  as  much  of  the  space  as  lies  in  front  of  the 
iris  is  called  the  anterior  chamber;  while  the  remaining  part, 
forming  a  slight  interval  between  the  back  of  the  iris  and 
the  lateral  part  of  the  lens,  is  distinguished  as  the  posterior . 
chamber. 


Fig.  119. — DEVELOPMENT  OP  THE  EYE,  a  diagram,  <z,  Cuticular 
epithelium;  b,  lens  developed  by  invagination  of  cuticle;  C9 
entrance  from  the  cavity  of  the  brain  into  the  primary  optic 
vesicle ;  d,  secondary  optic  vesicle ;  e,  e,  pia  mater ;  /,  choroid 
coat ;  g,  retinal  artery  entering  at  the  bottom  of  the  cleft  of  the 
eye;  h,  cerebral  substance  continued  into  the  optic  nerve  and 
retina ;  i,  epithelium  of  cerebral  cavity  continued  into  the  pig- 
mentary epithelium  of  the  choroid ;  k,  the  same  continued  into 
Jacob's  membrane. 

173.  Reviewing  the  whole  structure  of  the  eyeball,  it  may  be 
interesting  to  the  student  to  know  that,  in  the  early  embryo, 
it  is  developed  partly  from  the  integument  and  partly  from 
the  brain.  The  lens  is  originally  an  invagination  of  the 
skin,  which  becomes  converted  into  a  closed  sac;  and  its 
fibres  may  be  fairly  considered  as  elements  of  the  same  series 
as  the  elongated  cells -of  the  deepest  stratum  of  the  cuticle. 


236  ANIMAL   PHYSIOLOGY. 

The  optic  nerve,  the  retina,  and  the  choroid,  take  rise  from  a 
vesicular  outgrowth  of  the  brain,  comparable  with  the  olfac- 
tory bulb  (p.  204),  and  called  the  primary  optic  vesicle.  The 
neck  of  this  vesicle  remains  as  the  optic  nerve,  while  the 
distal  half  of  the  vesicle  becomes  invaginated  from  below 
upwards  and  backwards  against  the  other  half,  so  as  to  form 
with  it  a  double  cup,  the  secondary  optic  vesicle,  with  a  cleft 
in  its  lower  part.  The  pia  mater  or  vascular  covering  in 
the  half  nearest  to  the  optic  nerve  is  developed  into  the 
choroid,  and  in  the  invaginated  half  remains  as  the  retinal 
artery;  while  the  nervous  matter  of  the  first-mentioned  part 
disappears,  and  that  of  the  invaginated  portion  is  the  main 
substance  of  the  retina.  "Viewed  in  this  light,  the  bacillary 
layer  and  the  hexagonal  pigment  cells  of  the  choroid  are 
epithelial  developments  lining  the  opposed  surfaces  of  the 
optic  vesicle.*  The  vitreous  humour,  sclerotic,  cornea,  and 
iris,  are  later  developments  from  subcutaneous  tissue. 

174.  The  eye  may  be  likened  to  a  camera  obscura,  such  as 
that  which  is  used  in  photography.  In  front  are  the  refrac- 
tive media  by  which  the  inverted  image  is  produced,  while, 
behind,  the  retina  receives  that  image  precisely  as  the  ground 
glass  or  the  sensitive  plate  in  the  artist's  camera  receives, 
when  the  focus  is  rightly  adjusted,  the  picture  of  the  object 
to  be  photographed. 

It  must  not,  however,  be  supposed  that  the  eyes  of  all 
animals  have  the  complexity  of  the  eyes  of  vertebrate  animals, 
nor  that  vision  is  a  sense  enjoyed  in  perfection  by  all  animals 
possessed  of  eyes.  The  simplest  forms  of  eyes  met  with,  or 
of  structures  which  may  be  taken  for  eyes,  are  little  more 
than  spots  of  colour;  but  even  if  we  keep  all  doubtful  struc- 
tures out  of  view,  eyes  must  be  divided  into  those  which  are 
capable  of  receiving  an  image  of  the  landscape  and  those 
which  are  not.  A  scallop  (Pecteri)  is  provided  with  numerous 
eyes,  disposed  like  a  double  row  of  jewels,  but  it  cannot 
distinguish  objects.  Neither  can  the  starfish  do  so,  although 
it  has  a  group  of  eyes  at  the  tip  of  every  arm.  In  each  of 
these  eyes  there  is  a  dense  transparent  structure  or  lens  in 

*  For  tins  reason  I  object  to  considering  the  hexagonal  pigment 
cells  as  belonging  to  the  retina,  as  is  done  by  Max  Schultze,  who  lias 
done  so  much  to  elucidate  retinal  structures. 


VISION. 


237 


front,  with  nerve-terminations  behind  it,  and  bright  pig- 
ment round  about :  of  the  light  entering  the  lens,  no  doubt 
the  rays  corresponding  in  colour  with  the  pigment  are 
reflected  from  point  to  point,  and  a  sensation  must  thus  be 
produced  varying  in  intensity  with  the  amount  of  that  parti- 
cular colour  of  light;  but  it  can  scarcely  be  supposed  that  any 
nearer  approach  to  vision  is  made. 


Fig.  120.  —  OCELLI  OF  STARFISH   (solaster  papposa).      a,  Pignient- 
cone;  6,  lens;  c,  c,  nerve-corpuscles.     H.  S.  "Wilson. 

Eyes  which  receive  an  image  are  divisible  into  two  great 
groups,  those  in  which  the  image  is  erect,  and  those  in  which 
ifc  is  inverted.  To  the  first  of  these 
groups  belong  the  eyes  of  insects  and  of 
crustaceans,  such  as  lobsters;  while  to 
the  second  group  belong  the  eyes  of  ver- 
tebrata  and  cuttlefishes.  If  the  eye  of  a 
lobster  or  a  dragon-fly  be  carefully  ex- 
amined, it  will  be  seen  to  consist  of 
numbers  of  minute  facets,  barely  visible, 
crowded  together;  each  of  these  is  a 
transparent  structure  placed  at  the  ex- 
tremity of  a  long  tube  lined  with  black 
pigment,  and  with  a  nerve-termination  Fig.  121.— Diagram 
at  its  deep  extremity.  Obviously  no  of  INSECT'S  EYE  in 
ray  of  light  can  reach  the  bottom  of  section, 
such  a  tube  unless  it  fall  vertically  into  it.  Thus  the 
point  in  the  landscape  vertically  opposite  each  tube  affects 
the  nerve  at  its  extremity,  and  a  separate  sensation  is  pro- 
duced by  as  many  points  as  there  are  facets  in  the  eyes,  and 
this  will  happen  irrespective  of  the  distances  of  objects. 

In  eyes  which  invert  the  image,  the  inversion  is  produced  by 


238  ANIMAL  PHYSIOLOGY. 

the  addition  of  lens  and  camera,  which  are  probably  necessi- 
tated by  the  enormous  increase  in  number  of  the  sensitive 
points;  for  every  rod  or  cone  of  the  bacillary  layer  is  a 
separate  nerve-termination.  In  the  eye  of  the  cuttlefish,  the 
bacillary  layer  is  the  part  of  the  retina,  turned  towards  the 
lens,  and  is,  probably,  like  the  lens,  a  development  of  the 
cuticle;  but  in  the  vertebrate  eye,  we  have  seen  that  it  is 
the  part  of  the  retina  lying  against  the  choroid,  and  is  a 
development  of  the  brain. 

175.  If  the  analogy  of  nerve-terminations  in  other  organs 
be  attended  to,  it  will  be  at  once  perceived  that  those  of  the 
retina  are  the  rods  and  cones  of  the  bacillary  layer  ;  and  a 
variety  of  other  considerations  show  that  they  really  are  so. 
One  might  naturally  expect  that  the  surface  of  the  retina 
which  is  turned  towards  the  light,  would  be  the  one  to  be 
affected  by  the  rays  impinging  on  it  ;  but  this  is  not  the  case. 
For  the  part  next  the  light  is  the  layer  of  ramifying  fibres  of 
the  optic  nerve,  and  the  spot  where  those  fibres  are  most 
numerous  is  the  optic  pore,  which  happens  to  be  wholly 
insensible  to  light. 

Ihis  insensibility  of  the  optic  pore  can  be  easily  proved. 
The  axis  of  the  eye  is  always  directed  to  the  object  looked 
at;  in  that  axis  lies  the  yellow  spot,  and  to  its  inner  side 
is  the  optic  pore,  receiving  the  rays  entering  from  a  point 
external  to  the  object  looked  at.  If,  now,  the  left  eye 
be  shut,  and  the  right  eye  fixed  on  the  cross  here 
represented,  while  the  book  is  slowly  moved  towards  and 
away  from  the  eye,  at  a  certain  distance  the  round  mark  to 


the  right  will  suddenly  disappear,  to  come  again  into  view, 
as  the  book  is  brought  nearer  or  held  further  off.  The  same 
result  is  obtained,  if  the  experiment  be  made  with  white 
spots  on  a  dark  ground,  or  with  colours;  and  the  explana- 
tion obviously  is  that,  at  a  certain  distance,  the  round  mark 
falls  on  a  spot,  internal  to  the  axis,  which  is  insensible  to  the 
presence  or  absence  of  light. 

This  experiment  also  shows  the  importance  of  the  yellow 
spot  as  the  seat  of  clear  vision,  that  spot  from  which  the 
fibres  of  the  optic  nerve  are  absent,  and  in  which  the  rods  of 


VISION.  239 

the  bacillary  layer  are  entirely  replaced  by  tlie  more  highly 
organized  cones.  It  will  be  noticed  that  when  the  eye  is 
fixed  on  the  cross,  not  only  is  that  mark  the  most  distinctly 
visible  object,  but  letters  as  far  away  from  it  as  the  position 
of  the  round  ball,  although  still  a  long  distance  removed 
from  the  circumference  of  the  field  of  vision,  are  so  vaguely 
seen  that  they  cannot  be  distinguished  when  the  ball  is  hid 
from  view,  even  though  the  figure  is  so  arranged  that  this 
happens  when  the  book  is  held  at  a  good  focal  distance.  But 
the  optic  pore,  it  will  be  recollected,  is  only  one-tenth  of  an 
inch'  from  the  axis  of  the  eye,  therefore  this  observation 
shows  that  the  retina,  at  the  circumference  of  a  circle  with 
a  radius  of  a  tenth  of  an  inch,  and  the  axis  of  the  eye  as  its 
centre,  has  much  less  acute  sensibility  than  the  yellow  spot. 
And  inasmuch  as  there  is  no  interruption  of  the  outline  of 
the  field  of  vision  corresponding  to  the  position  of  the  optic 
pore,  but  the  place  where  such  an  interruption  might  be 
expected  to  show  itself  is  filled  up  with  the  appearance  round 
about,  whether  light,  dark,  or  coloured,  there  is  evidence  that 
the  sensation  originating  in  each  cone  or  rod  is  not  strictly 
limited;  although  in  other  parts  of  the  retina  it  is  practically 
limited  by  the  sensations  from  the  rods  round  about.  This 
explains  the  spreading  of  the  appearance  of  light  round  exces- 
sively luminous  objects. 

It  being  clear  that  the  layer  of  nerve  fibres  is  not  the  part 
t)f  the  retina  in  which  the  rays  of  light  produce  nervous 
impression,  it  is  curious  to  observe  that  the  rays  have  to 
pierce  these  fibres  and  the  strata  of  the  retina  before  reach- 
ing the  rods  and  cones  on  which  they  act.  This  shows  that 
the  susceptibility  to  a  stimulus  so  fine  as  light  depends  not 
on  the  nervous  structure  but  on  the  peculiar  terminal  organ 
added  thereto. 

176.  The  most  important  of  the  structures  through  which 
the  rays  of  light  pass  on  their  way  to  the  retina  is  the  crystal- 
line lens.  It  is  comparable  with  a  biconvex  lens  of  glass 
made  by  an  optician.  The  rays  of  light  from  each  visible  point 
in  the  landscape  pass  through  the  whole  aperture  of  the  pupil, 
and  are  refracted  towards  one  another  as  they  enter  the 
anterior  convex  surface  of  the  dense  substance  of  the  lens, 
and  again  as  they  emerge  from  its  convex  surface  behind. 


240  ANIMAL   PHYSIOLOGY. 

They  are  thus  gathered  together  at  a  certain  distance  behind 
the  lens,  in  a  focus  situated  in  the  direction  of  a  line  proceed- 
ing through  the  centre  of  that  body,  from  the  visible  point; 
and  the  rays  from  every  point  being  in  like  manner  refracted, 
an  inverted  image  of  the  landscape  is  produced. 


Fig.  122. — Diagram  to  illustrate  the  course  of  two  cones  of  liglit  to 
be  focused  on  the  retina,  and  that  distinct  vision  requires  that 
the  focus  for  the  object  looked  at  correspond  with  the  position 
of  the  retina. 

But  there  are  two  sets  of  conditions  required  for  the  pro- 
duction of  a  correct  image  on  the  retina :  one  is,  that  all  the 
rays  from  each  spot,  and  their  constituent  colours,  shall  be 
gathered  by  the  crystalline  lens  and  other  refracting  media 
quite  to  a  point;  the  other,  that  the  position  of  the  retina 
shall  correspond  with  the  focus. 

As  regards  the  first  of  these  conditions,  it  will  be  remem- 
bered, in  the  first  place,  that  only  parabolic  surfaces  have  the 
property  of  bringing  all  the  rays  from  each  point  to  a  perfect 
focus,  and  that  an  ordinary  glass  lens  has  an  imperfection 
dependent  on  its  spherical  curves,  termed  spherical  aberration. 
The  crystalline  lens  has  its  surfaces  approaching,  probably, 
pretty  near  to  the  parabolic  form,  but  it  must  not  be  for- 
gotten that  it  is  not  the  sole  refracting  medium  in  the  eye. 
Thus,  persons  who  have  had  the  crystalline  lens  extracted 
for  the  disease  called  cataract,  have  still  an  inverted  image 
thrown  on  the  retina,  and  continue  to  see,  although  imper- 
fectly, without  the  use  of  spectacles ;  thus,  also,  the  curvature 
of  the  cornea,  when  too  great,  produces  short-sightedness,  by 
making  the  refractive  apparatus  too  powerful.  It  is,  there- 
fore, proper  to  note  that  it  is  a  law  of  spherical  aberration, 
that,  while  it  is  exhibited  very  greatly  in  the  rays  which 
pass  through  the  lateral  parts  of  a  lens,  it  scarcely  affects 


VISION*.  241 

those  which  fall  vertically  near  its  centre;  and  therefore 
the  iris,  by  diminishing  the  pupil,  prevents  any  such  source 
of  error. 

Another  difficulty  in  the  manufacture  of  optical  instru- 
ments is  to  make  them  achromatic ;  that  is  to  say,  to  prevent 
the  different  colours  of  which  white  light  is  made  up  from 
being  dispersed  in  passing  through  lenses,  and  so  producing 
rainbow  colouring  in  the  image.  The  mechanician  meets 
this  difficulty  by  combining  lenses  of  contrary  form  and 
different  material,  in  such  a  manner  that  the  dispersion  of  a 
converging  lens  is  counteracted  by  the  opposite  and  equal 
dispersion  of  a  diverging  lens  of  less  refracting  power.  The 
same  expedient  is  resorted  to  in  the  structure  of  the  eye,  the 
light  having  to  pass  successively  through  the  cornea,  aqueous 
humour,  and  the  different  strata  of  unequal  density  in  the 
crystalline  lens. 

177.  With  reference  to  the  correspondence  of  the  position  of 
the  retina  with  the  focus  of  the  lens,  it  will  be  noticed  that 
if  that  membrane  be  so  placed  that  the  images  of  distant 
objects  shall  be  cast  on  it,  it  will  be  too  far  forward  to  receive 
the  images  of  near  objects;  and,  vice  versd,  if  it  be  so  situated 
as  to  receive  the  images  of  near  objects,  it  will  be  too  far  back 
for  those  at  a  distance,  unless  some  arrangement  of  accom- 
modation be  specially  brought  into  play.  In  a  photographer's 
camera,  the  focus  is  arranged  for  different  distances  by  moving 
the  lens  forwards  and  backwards;  in  the  eye,  the  same 
object  is  attained  by  change  in  the  form  of  the  lens. 

It  is  easy  to  make  certain  that  a  change  of  some  sort  takes 
place  in  the  eye  to  accommodate  it  to  different  distances. 
One  has  only  to  shut  one  eye,  and  hold  up  a  finger  a  few 
inches  from  the  other,  to  perceive  that,  if  the  finger  be 
steadily  looked  at,  the  background,  even  at  the  other  end  of 
the  room,  is  quite  indistinct;  and,  as  soon  as  by  an  effort  of 
the  will  the  sight  is  fixed  on  the  background,  the  finger  in 
turn  loses  all  distinctness  of  outline.  But  it  is  more  difficult 
to  determine  the  nature  of  the  change.  It  has  been  dis- 
covered by  careful  observation  of  the  reflections  from  the 
surfaces  of  the  lens  (Helmholtz).  "When  a  light  is  held  in 
front  of  an  eye,  three  images  are  reflected;  one  is  from  the 
surface  of  the  cornea;  another,  of  a  dim  description,  is  from 
H  Q 


242 


ANIMAL   PHYSIOLOGY. 


the  anterior  surface  of  the  lens;  and  a  third,  small  and  clear, 
and  inverted,  is  from  the  hinder  surface  of  the  lens.  When 
the  focus  of  the  eye  is  changed  from  distant  to  near  objects, 
without  altering  the  direction,  the  inverted  image  retains  its 
form  and  position,  while  that  from  the  front  of  the  lens 
becomes  smaller,  and  approaches  the  corneal  image.  From 
this  it  is  known  that  the  posterior  surface  of  the  lens  remains 
unchanged,  while  the  anterior  surface  is  made  more,  con  vex; 
and  by  examination  of  the  eye  in  profile,  the  lens  has  even 
been  seen  to  project  through  the  pupilary  aperture  when 
adjusted  to  short  distances.  The  cause  of  this  change  of 
shape  is  not  thoroughly  understood,  but  the  principal  agent 
in  effecting  it  would  appear  to  be  the  ciliary  muscle. 


Fig.  123. — ACCOMMODATTON  TO  DISTANCES.  F,  Lens  accommodated  to 
far  objects ;  N,  to  near  objects ;  a,  anterior  chamber ;  6,  posterior 
chamber ;  c,  canal  of  Petit ;  d,  ciliary  muscle ;  e,  ciliary  process. 

Another  change  which  takes  place  in  adjustment  to  short 
distances  is  lessening  of  the  pupil;  and  it  has  been  suggested 
that  the  contraction  of  the  circular  fibres  of  the  iris  presses 
on  the  sides  of  the  lens  so  as  to  alter  its  form.  But  this 
supposition  is  disproved,  not  only  by  the  fact  that  the  faculty 
of  adjustment  has  been  found  unimpaired  when  the  iris  has 
been  wanting,  but  by  the  size  of  the  pupil  being  diminished 
still  more  by  increase  of  light  than  by  looking-at  near  objects. 
The  iris  seems  to  act  simply  as  a  diaphragm,  cutting  off  the 
lateral  rays;  and  this  is  specially  required  in  a  bright  light, 
to  save  the  retina  from  undue  stimulation,  and  in  looking  at 
near  objects,  because  the  rays  from  them  are  so  exceedingly 
divergent. 

17S.  When  objects  are  looked  at  with  both  eyes,  the  muscles 
which  move  the  eyeballs  are  brought  into  requisition.  These 


vision.  243 

are  four  recti  and  two  obliqui  muscles.  Tlie  recti  come  for- 
wards from  tlie  back  of  the  orbit,  to  be  inserted,  in  front  of 
the  middle  of  the  eyeball,  into  the  sclerotic;  and  they  are 
named  from  their  positions,  superior,  inferior,  external,  and 
internal.  The  superior  oblique  muscle  passes  forwards  from 
the  back  of  the  orbit  to  the  inner  and  upper  angle  of  its  fore 
part ;  there,  becoming  tendinous,  it  passes  through  a  pulley  of 
fibres  attached  to  the  frontal  bone,  and,  changing  its  direction, 
turns  outwards  and  backwards  to  be  attached  to  the  outer 
part  of  the  eyeball,  behind  the  middle.  The  inferior  oblique 
muscle,  springing  from  a  point  at  the  lower  part  of  the  inner 
margin  of  the  front  of  the  orbit,  takes  a  similar  direction  to 
the  tendon  of  the  superior  oblique,  passing  backwards  and 
outwards  below  the  eyeball,  to  be  inserted  on  its  outer  side. 


Fig.  124. — MUSCLES  OF  THE  EYEBALL.  «,  Optic  nerve;  b,  superior 
oblique  muscle ;  c,  pulley  for  the  tendon  of  the  same ;  d,  inferior 
oblique  muscle.  The  other  four  muscles  are  the  four  recti. 

The  superior  recti  muscles  of  the  two  eyes  always  act  in 
concert,  as  also  do  the  inferior  recti,  but  the  external  and 
internal  recti  act  differently  in  different  circumstances.  In 
turning  the  head,  while  the  eyes  are  fixed  on  a  stationary 
object,  or  in  following  an  object  which  crosses  in  front  of  us, 
the  external  rectus  of  one  eye  acts  in  concert  with  the  internal 
rectus  of  the  other;  but,  in  turning  the  eyes  from  a  distant 
to  a  near  object,  we  make  their  axes  converge  on  the  object 
looked  at,  and  the  internal  recti  of  both  eyes  act  together. 
These  limitations  of  the  movements  of  muscles  which  must 
be  considered  voluntary  are  exceedingly  curious,  and  find  no 
parallel  in  the  muscles  of  the  limbs. 

The  use  of  the  oblique  muscles  is  not  at  first  obvious;  but 
it' will  be  observed  that  when  the  eyes  are  converged,  the 


244  ANIMAL   PHYSIOLOGY. 

superior  and  inferior  recti,  acting  by  themselves,  will  be  no 
longer  capable  of  rotating  them  directly  upwards  and  down- 
wards, seeing  that  the  axis  of  vision  and  the  direction  of 
these  muscles  are  no  longer  in  the  same  vertical  plane;  and 
the  oblique  muscles  are  so  disposed  that,  by  acting  in  concert 
with  the  superior  and  inferior  recti,  they  are  capable  of  com- 
pensating for  the  deviation  of  the  direction  of  these  muscles 
from  the  axis  of  vision,  and  so  maintain  the  vertical  diameters 
of  the  eyeballs  parallel  to  one  another,  which  we  shall  im- 
mediately see  to  be  a  condition  necessary  for  perfect  vision. 

179.  When  we  look  at  an  object  with  both  eyes  directed  full 
upon  it,  we  see  it  as  a  single  object,  notwithstanding  that  two 
images  of  it  are  received,  one  on  each  retina.  But  by  arti- 
ficial expedients  these  two  images  may  be  made  to  give  the 
appearance  of  two  objects;  that  is  to  say,  double  vision  may 
be  produced.  By  pressing  a  finger  gently  on  the  side  of  one 
eyeball,  so  as  to  derange  the  position  of  its  axis  of  vision,  a 
second  picture  of  each  ol^ject  in  the  landscape  may  be  made 
to  appear,  either  above,  below,  or  to  one  side  of  the  more 
distinct  picture  presented  to  the  other  eye,  according  to  the 
direction  of  pressure.  Or,  if  a  finger  be  held  up  exactly  in 
front  of  a  more  distant  object,  and  the  eyes  be  directed  to 
the  finger,  while  the  attention  takes  cognisance  of  the  object 
beyond,  that  object  will  be  seen  double.  On  the  other  hand, 
in  looking  through  a  stereoscope,  two  pictures  have  the 
appearance  of  one.  All  these  phenomena  depend  on  one 
law,  which  may  be  expressed  thus :  that  rays  which  fall  on 
points  in  the  outer  half  of  one  retina,  are  referred  by  the 
mind  to  the  same  direction  as  those  which  fall  on  similarly 
situated  points  in  the  inner  half  of  the  other  retina.  Such 
points  are  therefore  said  to  be  physiologically  corresponding 
or  identical.  When  one  eye  is  moved  from  its  position  by 
pressure  of  a  finger  on  it,  the  different  points  of  the  land- 
scape are  no  longer  thrown  on  identical  points  of  the  retinse, 
and  they  are  therefore  seen  double.  When  the  eyes  converge 
on  an  object  in  front  of  another,  the  images  of  the  hinder 
object  are  thrown  on  the  inner  halves  of  both  retinae,  and 
therefore  on  points  not  identical.  But,  in  looking  through 
the  stereoscope,  although  the  pictures  are  two,  each  eye  is 
directed  full  on  the  picture  opposite  it,  and  thus  correspond- 


vision.  245 

ing  parts  of  the  pictures  are  thrown  on  identical  points,  and 
referred  to  identical  positions  in  space. 

Double  vision,  it  may  here  be  mentioned,  can  be  likewise 
artificially  produced  when  only  one  eye  is  used.  The  prin- 
ciple, however,  is  different  from  that  which  we  have  been 
considering.  If,  in  a  card,  a  few  pin  holes  be  made  so  close 
together,  that  they  shall  be  within  a  space  not  larger  than  the 
aperture  of  the  pupil,  and  the  pin  holes  be  held  to  the  eye, 
objects  at  some  distance  will  be  seen  perfectly;  but  a  minute 
object,  such  as  a  pin  head,  held  near  the  card  will  appear  mul- 
tiplied as  many  times  as  there  are  pin  holes.  The  explanation 
is,  that  the  pin  head  is  out  of  focus,  and,  looked  at  without 
a  diaphragm,  would  be  invisible;  but  the  diaphragm  cuts  off 
a  large  part  of  each  of  the  pencils  of  rays  which  spread  out 
towards  the  pupil  and  would  have  been  diffused  over  an 
area  of  the  retina  so  as  to  interfere  with  one  another;  and 
the  perforations  admit  only  very  small  portions  of  them, 
which  fall  on  different  parts  of  that  area,  and  are  so  narrow 
that  the  deficiency  in  focus  is  not  sufficient  to  produce  con- 
fusion. 

180.  Physiologists  have  sometimes  exercised  their  ingenuity 
in  trying  to  account  for  our  seeing  objects  erect,  notwithstand- 
ing that  the  pictures  on  the  retina  are  inverted.     But  a  little 
reflection  will  show  that  the  inversion  of  the  retinal  image 
is   no  reason  why  the  landscape  should    appear  inverted. 
What  we  perceive  is  not  the  retinal  image,  but  a  number  of 
sensations  excited  by  it;  and  it  must  be  considered  as  an 
ultimate  fact,  that  the  sensation  produced  by  irritation  of  a 
rod  or  cone  of  the  retina  is  not  perceived  as  being  in  that 
structure,  but  as  situated  vertically  opposite  it,  outside  the 
body.     If  we  are  to  explain  why  the  landscape  is  not  seen 
inverted,  we  must  explain  why  it  is  not  seen  inside  our  heads. 
A  child  does  not  rectify  an  inverted  landscape  by  experience 
derived  from  touch,  any  more  than  it  imagines  the  external 
world,  as  manifested  by  vision,  to  be  concentrated  in  two 
small  spots  at  the  bottom  of  its  eyeballs. 

181.  Distance,  however,  is  a  thing  which  the  eye  certainly 
learns  to  appreciate  by  experience.    A  child,  from  its  entrance 
into  the  world,  no  doubt  sees  objects  as  things  outside  itself; 
but  it  learns  only  by  practice  the  distances  of  different  objects. 


246  ANIMAL  PHYSIOLOGY. 

This  might  be  gathered  from  watching  the  movements  of 
infants;  but  it  has  been,  more  distinctly  demonstrated  by 
observations  made  on  persons  born  blind,  who  have  gained 
sight  after  some  years,  by  a  surgical  operation.  Such  persons 
see  every  thing  at  first  as  if  close  at  hand,  and,  from  not 
understanding  the  effects  of  distance,  form  most  erroneous 
ideas  of  the  sizes  of  objects;  and  they  handle  things  when 
they  look  at  them,  so  as  to  compare  the  results  of  vision  with 
those  of  common  sensation.  The  effects  of  distance  on  the 
eyes,  which  experience  teaches  us  to  translate,  are  of  various 
descriptions.  (1)  The  distance  of  the  object  looked  at 
determines  the  degree  of  convergence  of  the  eyes;  (2)  it 
determines  the  focus;  (3)  it  affects  the  intensity  of  light 
and  shade,  and  the  colour,  producing  what  is  called  per- 
spective of  colour;  (4)  it  diminishes  the  apparent  size  of 
objects,  producing  linear  perspective;  so  that  when  from 
custom  or  otherwise  the  size  of  an  object  is  known,  its 
distance  is  estimated.  That  the  convergence  of  the  eyes  is 
of  some  use  in  enabling  us  to  appreciate  the  exact  distance 
of  near  objects,  is  illustrated  by  the  difficulty  which  one  has 
in  threading  a  needle  when  one  eye  is  shut.  But  by  a  little 
practice  that  difficulty  is  overcome,  which  shows  that  the 
use  of  two  eyes  is  not  essential  to  judging  distances. 

182.  The  appearances  of  solidity  and  hollowness  depend 
partly  on  the  apparent  diminution  of  receding  objects,  partly 
on  the  way  in  which  the  light  falls,  and  partly  on  the  pictures 
presented  to  the  two  eyes  being  different,  and  bringing  into 
view  a  larger  amount  of  surface  than  can  be  seen  from  one 
point.  The  last  of  these  three  causes  is  supplemented 
materially  by  the  other  two;  otherwise  the  appearance  of 
solidity  would  be  lost  on  shutting  one  eye.  It  is,  however, 
a  most  important  element,  as  is  shown  by  the  effects  of  the 
stereoscope,  when  geometrical  figures  are  looked  at. 

I  have  already  pointed  out  how  it  is  that  only  one  figure 
is  seen  when  two  pictures  are  looked  at  through  the  stereo- 
scope. But  stereoscopic  effect  depends  on.  the  circumstance 
that  no  solid  or  hollow  body  presents  exactly  the  same  view- 
to  both  eyes.  The  artist  provides  on  the  stereoscopic  slide 
two  views  of  one  object,  such  as  would  be  presented  in  nature 
to  the  two  eyes;  and  the  eyes  are  directed  by  the  construction 


247 

of  the  instrument,  so  that  nearly  similar  parts  of  the  pictures 
fall  on  identical  points  of  the  retina.  In  looking  at  a  solid 
object,  the  portions  of  its  sides  brought  into  view  are  seen 
to  a  greater  extent  by  the  eye  on  the  same  side  than  by  the 
other;  but  in  a  hollow  object,  they  produce  the  broader  image 
in  the  eye  on  the  opposite  side ;  and  thus  it  happens  that  if 
the  stereoscopic  views  of  a  solid  body  be  clipped  separate, 
raid  each  be  placed  in  the  instrument  in  the  position  which 
was  intended  for  the  other  figure,  it  is  made  to  appear  as 
a  hollow.  This  effect  can  be  obtained  in  perfection  with 
geometrical  figures  without  shading;  but  is  aided  by  the 
reversal  of  the  shading  when  an  irregular  figure  is  looked  at. 
By  means  of  another  instrument,  the  pseudoscope,  the  rays 
coming  from  actual  objects  are  directed  in  such  a  manner 
that  the  image  which  should  be  presented  to  one  eye  is  made 
to  fall  on  the  other,  and  by  this  means  raised  objects  seem  as 
if  hollow,  and  vice  versd.  But  the  important  part  which 
experience  plays  in  giving  the  idea  of  solidity  and  hollow- 
ness,  is  shown  by  the  circumstance  that  neither  with  stereo- 
scope nor  pseudoscope  can  the  reversal  of  appearance,  or 
conversion  of  relief,  as  it  is  termed,  be  obtained  when  the 
objects  looked  at  are  of  a  complex  description,  and  so  appeal 
to  our  associations  that  they  cannot  be  conceived  otherwise 
than  as  they  really  are.. 

183.  It  has  already  been  pointed  out  that  impressions  on 
the  retina  have  a  certain  tendency  to  diffusion;  they  have 
likewise  a  tendency  to  endure  after  removal  of  the  stimulus, 
particularly  if  this  have  been  applied  with  much  intensity, 
or  for  a  length  of  time.  After  gazing  at  the  sun,  or  any 
bright  light,  a  spectrum  or  coloured  figure  remains  before  the 
eye  for  some  time;  and  any  object,  whether  brilliant  or  not, 
if  moved  rapidly,  can  be  shown  to  leave  impressions  on  the 
retina  which  endure  after  cessation  of  the  stimulus.  When 
a  wheel  of  a  carriage  is  in  motion,  the  spokes  become  indistin- 
guishable one  from  another,  and  a  dull  tinge  of  their  colour, 
brightest  near  the  nave,  is  diffused  round  about;  because  the 
spokes  each  affect  the  retina  in  every  part  of  their  revolution, 
the  impression  made  by  one  at  any  part  being  immediately 
succeeded  by  another,  and  the  spokes  are  placed  most  closely 
together  near  the  centre,  This  imperfection  of  vision  would 


248  ANIMAL   PH73IOLOGY. 

be  much,  greater,  were  ifc  not  that  the  eye  has  a  tendency  to 
follow  any  object  on  which  the  gaze  is  fixed.  The  eye  follows 
the  wheel  of  a  carriage,  and  the  image  continuing  to  be  made 
on  one  part  of  the  retina  is  correctly  appreciated  as  circular ; 
but  it  fails  to  follow  the  spokes,  and  therefore  these  affect 
successive  parts  of  the  retina  with  great  rapidity.  The  dura- 
tion, of  retinal  impressions  is  the  principle  on  which  a  number 
of  optical  toys  depend.  The  simplest  of  them  are  cards  with 
pictures  on  each  side,  and  twirled  round  by  a  couple  of 
strings,  one  at  each  end,  so  as  to  bring  the  pictures  together. 
Thus,  a  bird  on  one  side  and  a  cage  on  the  other  gives  the 
appearance  of  a  bird  within  a  cage ;  and  a  man  riding  a  horse 
may  be  brought  into  view,  the  man  being  painted  on  one 
side  of  the  card  arid  the  horse  on  the  other.  The  thaumotrope 
and  the  anorthoscope  are  instances  of  much  more  complex 
toys  dependent  011  the  same  principle  (see  Glossary). 

The  coloured  spectra  seen  after  gazing  on  bright  objects  are 
connected  with  something  more  than  the  mere  duration  of 
impressions,  namely,  the  power  of  appreciating  different 
colours.  The  appreciation  of  colour  is  not  understood;  we 
do  not  know  by  what  mechanism  the  rods  and  cones  are 
thrown  into  different  conditions  by  lights  of  equal  intensity 
but  different  wave  lengths,  so  that  both  colour  and  intensity 
are  appreciated  by  one  set  of  structures.  But  it  is  known 
that  the  appreciation  of  colour  is  a  separate  power  from  the 
appreciation  of  light  and  shade,  for  there  are  various  kinds  of 
colour-blindness  which  occur  uncomplicated  with  other  defect 
of  sight. 

184.  Colour-blindness  is  of  three  sorts.  Some  persons  have 
been  found  unable  to  appreciate  any  difference  of  colour  at  all, 
to  whom  the  world  was  like  an  engraving.  A  number  of  persons 
have  an  inability  to  distinguish  allied  tints  one  from  another, 
and  confuse  blue  with  green,  or  confound  pinks,  crimsons, 
and  scarlets  together.  In  a  third  set  of  cases,  colours  totally 
unlike  are  confounded;  and  the  most  remarkable  colour- 
blindness of  this  description  consists  in  inability  to  distin- 
guish red  from  green  or  blue.  Curiously  enoiigh,  this  defect 
may  exist  without  the  object  of  it  having  any  suspicion  that 
his  vision  is  defective.  Thus,  Dalton,  the  celebrated  chemist, 
from  whom  colour-blindness  occasionally  gets  the  name  of 


Vision.  249 

Daltonism,  was  twenty-five  years  of  age  before  lie  was  dis- 
tinctly convinced  of  his  peculiarity  of  vision.  Yet  so  great 
was  this  peculiarity  that  in  describing  it  he  wrote :  "  Crimson 
appears  a  muddy  bkie  by  day,  and  crimson  woollen  yarn  is 
much  the  same  as  dark  blue;"  and  further  recorded  that  the 
one  side  of  a  laurel  leaf  seemed  to  him  a  good  match  to  a 
stick  of  sealing-wax,  and  the  other  side  to  a  red  wafer.  After 
Dalton  had  attracted  attention  to  the  matter,  it  was  found 
that  his  case  was  so  far  from  being  solitary  that  colour-blind- 
ness in  different  degrees  was  not  unfrequently  to  be  met  with. 
This  being  the  case,  it  is  evident  that,  on  railways  and  at  sea. 
danger  signals  dependent  on  the  number  or  position  of  lights 
are  preferable  to  those  dependent  on  colour,  and  that  red 
lights  are  specially  objectionable. 

185.  The  laws  which  regulate  the  colours  of  the  ocular  spec- 
tra, above  alluded  to,  are  curious,  and  not  so  easily  explained 
as  we  are  often  asked  to  believe.  If  a  brightly-coloured  object 
on  a  white  ground  be  steadily  gazed  at,  on  looking  away  from 
it  to  the  surface  on  which  it  lies,  or,  still  better,  looking  tc  a 
dark  surface,  or  shutting  the  eyes,  the  image  of  the  object 
remains  before  the  sight,  in  the  complementary  colour — that 
is  to  say,  in  the  colour  which,  added  to  that  of  the  object 
gazed  on,  would  make  white  light.  If  the  object  be  red;  the 
spectrum  will  be  green,  and  if  the  object  be  blue,  there  will 
be  an  orange  spectrum ;  and  the  explanation  commonly  given 
is,  that  the  part  of  the  retina  on  which  the  coloured  rays  have 
fallen  becomes  by  exhaustion  less  affected  by  the  rays  of  the 
same  colour  in  the  light  around,  while  it  is  affected  by  all  the 
colours  entering  into  white  light.  The  following  circum- 
stances, however,  show  that  explanation  to  be  insufficient : — 
1 .  The  brightest  complementary-coloured  spectrum  is  ob  tained 
by  shutting  the  eyes,  or  looking  into  total  darkness.  2. 
While  continuing  to  gaze  on  the  coloured  object,  a  ring  of 
light  more  brilliant  than  the  surrounding  surface  makes  its 
appearance  round  about,  and  the  spectrum  seen  on  the 
white  surface  is  of  the  same  shade  as  that  ring  of  light.  3. 
If  the  object  gazed  at  be  on  a  dark  ground,  the  ring  about 
it  will  be,  not  one  of  light,  but  of  greater  darkness;  and 
the  spectrum,  if  cast  on  that  ground,  will  be  of  the  same 
shade.  4.  If  a  very  brilliant  spectrum  be  obtained,  its  colour 


250  ANIMAL   PHYSIOLOGY. 

can  be  temporarily  changed  by  pressure  on  the  shut  eyes,  and 
will  again  return. 

186.  The  effects  of  pressure  on  coloured  spectra  afford  an 
interesting  illustration  of  the  fact  that  mechanical  irritation  of 
the  retina  produces  a  sense  of  light,  and  not  of  pain.  I  gaze 
on  the  name  of  the  lamp  beside  me,  and  on  shutting  my  eyes 
I  see  a  spectrum,  which,  instead  of  being  of  the  comple- 
mentary colour,  is  bright  yellow,  with  a  red  margin,  and  float- 
ing on  a  dark  green  halo.  I  press  my  fingers  against  my 
closed  eyes,  and  obtain  the  complementary  colour,  namely,  a 
violet  spectrum  with  a  green  margin  on  a  yellow  halo.  On 
removing  the  pressure  the  original  colours  return;  and  this 
can  be  repeated  several  times.  The  appearances  are  even 
more  complex,  if  the  experiment  be  made  with  sunlight. 

Phenomena  of  light  uncomplicated  with  colour  can  likewise 
be  obtained  by  mechanical  irritation.  An  accidental  blow 
on  the  eye  produces  an  appearance  of  sparks  of  fire;  and  by- 
gentle  pressure  the  effects  called  phosphenes  are  obtained. 
A  phosphene  is  a  luminous  image  produced  by  shutting  the 
eyes,  and  touching  one  of  them  lightly  but  firmly  on  the 
outer,  inner,  upper,  or  lower  border — in  short,  on  any  part 
where  the  retina  extends.  A  luminous  crescent,  or  complete 
circle,  flashes  into  sight  at  the  point  diametrically  opposite 
the  pressure.  This  is  called  the  larger  phosphene,  and  13 
caused  by  irritation  of  the  retina  at  the  point  touched,  re- 
ferred by  the  mind,  like  all  retinal  impressions,  to  the  posi- 
tion vertically  opposite.  Besides  this,  a  smaller  pliosphene 
may  be  obtained,  visible  at  the  part  touched,  which  is  caused 
by  the  contents  of  the  eyeball  being  pressed  against  the 
opposite  point.  The  smaller  phosphene  is  a  blush  of  light  of 
variable  intensity,  extending  over  a  space,  larger  or  smaller, 
according  to  the  size  of  the  object  with  which  pressure  is 
made;  the  larger  phosphene  is  always  brilliant,  evanescent, 
and  confined  to  a  ring.  Phosphenes  are  much  more  easily 
produced  at  one  time  than  another ;  and  after  reading  to  a 
late  hour,  the  mere  closure  of  the  eyelids  in  a  dark  room  may 
cause  a  bright  circle  of  light  to  flash  before  each  eye. 

By  means  of  pressure,  patterns  produced  by  a  number  of 
internal  structures  of  the  eyeball  can.  be  brought  into  view. 
The  branches  of  the  retinal  artery  may  thus  be  seen  as  dark 


VISION*.  25 1 

or  luminous  lines;  also  another  pattern,  which  appears  to 
be  the  network  of  the  choroidal  capillaries;  and  sometimes 
patches  of  small  points  closely  set  together,  which  have  exactly 
the  appearance  of  the  extremities  of  the  rods  and  cones  of  the 
retina  itself.  These  experiments  may  be  carried  to  the  extent 
of  giving  pain,  arid  are  certainly  bad  for  the  eyes. 

The  retinal  vessels,  however,  can  be  seen  in  a  less  unplea- 
sant way,  by  holding  a  light  a  few  inches  from  the  side  of  the 
eye  in  a  dark  room,  and  gazing  forwards  into  the  darkness 
while  the  light  is  gently  moved.  In.  a  little  while  the  field 
of  vision  becomes  yellowish,  and  dark  lines  are  seen  ramifying 
in  it  in  the  position  of  the  branches  of  the  retinal  artery. 
These  are  what  are  termed  figures  of  Purkinje,  and  are 
occasioned  by  the  vessels  intervening  between  the  light  and 
the  rods  and  cones  behind  them.  The  blood  corpuscles,  as 
they  traverse  the  anterior  layers  of  the  retina,  can  also  be 
seen  as  luminous  spots,  when,  one  gazes  intently  into  a  clear 
sky. 

187.  Lachrymal  apparatus.  —  The  secretion  of  tears  is 
primarily  useful  for  keeping  the  surface  of  the  cornea  clear, 
and,  in  connection  with  this  object,  there  are  channels  pro- 
vided by  which  they  may  be  removed  without  unduly  accu- 
mulating or  rolling  over  the  cheeks.  But  the  flow  of  tears  is 
likewise  influenced  by  the  emotions;  and  any  one  who  is 
familiar  with  the  important  effects  Avhich  may  be  wrought 
by  the  action  of  one  or  two  leeches,  will  be  slow  to  doubt 
that  a  copious  flood  of  tears  may  be  of  great  utility  in  reliev- 
ing a  congested  condition  of  the  brain. 

The  lachrymal  gland  is  similar  in  structure  to  the  salivary 
glands,  is  about  the  size  of  an  almond,  is  situated  in  the 
upper  and  outer  angle  of  the  orbit,  under  cover  of  the  upper 
eyelid,  and  pours  out  its  secretion  by  several  ducts. 

The  eyelids  are  closely  associated  with  the  lachrymal  gland 
in  function.  The  upper  lid  is  the  more  important  of  the 
two;  it  is  larger  than  the  lower,  has  twice  as  many  eyelashes, 
has  a  special  muscle  for  raising  it,  and  is  stiffened  by  a  cres- 
centic  plate  of  thin  cartilage  (superior  tarsal  cartilage) 
beneath  its  mucous  membrane,  while  the  lower  lid  has  only 
a  linear  strip  of  that  substance  inside  its  margin.  The 
upper  and  lower  tarsal  cartilages  are  united  at  the  inner 


252 


ANIMAL   PHYSIOLOGY. 


corner  or  canthus  of  the  eye  by  a  little  tendon  (tendo  oculi) 
to  the  bone;  and  to  the  sides  of  this  tendon  are  attached  the 
fibres  of  the  orbicularis  palpebrarum  muscle,  a  thin  sheet  of 
subcutaneous  fibres  which  pass  in  circles  round  the  eyelids, 
spreading  over  the  adjacent  parts  of  the  cheek  and  forehead. 
This  is  the  muscle  by  which  the  eyelids  are  closed;  and  its 
attachment  to  the  tendo  oculi  explains  why  it  is  that,  when 
the  lids  are  forcibly  shut,  as,  for  example,  by  a  reflex  action 
on  tasting  something  sour,  their  edges  are  drawn  inwards  to 
the  nose.  When  the  eyes  are  opened,  the  lower  lid  falls  back 
into  its  place  by  elasticity,  but  the  upper  lid  is  raised  by  its 
levator  palpebrce  muscle,  which  comes  forward  from  the  back 
of  the  orbit,  lying  on  the  superior  rectus,  and  is  attached  to 
the  upper  border  of  the  superior  tarsal  cartilage.  On  evert- 
ing either  eyelid,  a  set  of  nodulated  yellow  streaks  may  be 
seen  beneath  the  mucous  membrane  or  conjunctiva  (p.  229). 
They  are  the  Me  ibomian  follicles,  and  are  a  set  of  sebaceous 
glands  which  keep  the  margins  of  the  lids  oiled,  and  so  help 
to  prevent  the  tears  from  running  over  the  cheeks. 


Fig.  125. — LACHRYMAL  APPARATUS,  a,  Levator  palpebrae  muscle; 
&,  tarsal  cartilage  of  the  upper  eyelid;  c,  Meibomian  follicles, 
exhibited  by  division  of  the  upper  eyelid  and  reflection  of  the 
outer  part ;  d,  caruncula ;  e,  plica  semilunaris ;  /,  lachrymal  gland 
with  the  orifices  of  its  ducts  below  it;  #,  canaliculi,  with  the 
pimcta  lachrymalia  on  the  edges  of  the  eyelids;  h,  lachrymal 
sac  ;  «,  nasal  duct. 


HEARING.  253 

At  the  inner  canthus  of  the  eye,  there  is  a  little  spongy- 
looking  bit  of  mucous  membrane,  called  the  caruncula,  rest- 
ing on  a  fold  of  smoother  membrane,  the  edge  of -which  is 
laid  against  the  eyeball.  The  fold  is  the  plica  semilunaris, 
and  is  the  vestige  or  representative  of  a  third  eyelid,  more 
distinct  in  many  mammals  than  it  is  in  man,  and  developed 
in  birds,  especially  those  of  the  nocturnal  sort,  as  the  mem- 
brana  nictitans,  a  structure  which  rapidly  sweeps  across  the 
eyeball  to  clear  it,  instead  of  the  upper  lid  moving  as  it  does 
when  we  wink.  This  can  be  readily  seen  in  the  owl. 

Opposite  the  plica  semilunaris,  the  margins  of  the  eyelids, 
particularly  that  of  the  lower  lid,  change  their  direction;  and 
on  the  elevation  where  this  takes  place  there  may  be  seen  in 
each,  when  the  lid  is  slightly  pulled  outwards,  a  little  open- 
ing which  rests  against  the  eyeball.  These  openings  are  the 
puncta  lacJirymalia,  which  lead  into  two  minute  ducts  called 
canaliculij  each  of  which  passes  vertically  into  the  eyelid  for 
a  very  short  distance,  then  turns  abruptly  inwards  to  open 
into  a  sac  behind  the  tendo  oculi,  whence  the  tears  pass 
downwards  by  the  nasal  duct  (p.  37)  into  the  nose.  Thus 
the  tears,  secreted  by  the  lachrymal  gland,  pass  across  the 
eye,  washing  the  conjunctiva;  and  every  time  a  wink  takes 
place,  the  puncta  lachrymalia,  and  the  parts  of  the  canalicuH 
in  connection  with  them,  are  lightly  pressed  against  the  eye, 
so  that  when  the  pressure  is  removed,  the  moisture  is  sucked 
into  their  interior;  and  only  when  there  is  a  redundant 
secretion,  or  when  by  some  irregular  movement,  as  in  laughter, 
the  puncta  lachrymalia  are  disarranged,  do  the  tears  accu- 
mulate within  the  lids,  and  overrun  the  cheeks. 

188.  Hearing. — The  simplest  form  of  ear  may  be  studied 
with  advantage  in  the  cuttlefishes,  in  which  animals  the 
organs  of  hearing  are  imbedded  in  a  cartilaginous  collar  in 
the  neck,  and  consist  each  of  a  sac  supplied  with  nerves, 
filled  with  fluid,  and  containing  some  loose  particles  of  hard 
substance.  The  vibrations  of  sound  are  communicated  to 
the  fluid  in  the  sac,  and,  according  to  an  acoustic  law,  are 
strengthened  by  beating  against  the  solid  particles  contained 
in  it,  and  they  stimulate  the  nerve-terminations.  But  in  all 
vertebrate  animals,  the  ear  is  much  more  complex,  and, 
particularly  in  mammals,  not  only  is  the  primary  sac  highly 


25i 


ANIMAL    PHYSIOLOGY. 


complicated  to  produce  an  organ  capable  of  appreciating  the 
fine  varieties  of  sound,  but  there  are  many  accessory  parts 
added,  which  bring  sounds  within  reach  of  the  sensitive  struc- 
tures, and  likewise  protect  these  from  over-stimulation. 

The  human  ear  consists  of  three  parts :  the  external,  middle, 
and  internal  ear.  The  internal  ear  is  the  essential  organ  of 
hearing,  filled  with  fluid,  and  containing  the  distribution  of 
the  auditory  nerve.  The  external  and  middle  ears  contain 
air,  the  external  ear  being  open  to  the  outside,  and  the 
middle  ear  or  tympanum,  as  it  is  called,  communicating  with 
the  pharynx;  and  they  are  separated,  one  from  the  other,  by 
a  partition,  the  membrana  tympdni. 


Fig.  126.— CARTILAGE  AND  MUSCLES  OF  EXTERNAL  EAR.  A,  Outer 
aspect.  B,  Cranial  aspect,  a,  6,  c,  attrahens,  attollens,  and  retra- 
liens  auriculam  muscles ;  d,  concha ;  e,  antihelix ;  /,  g,  large  and 
small  muscle  of  helix ;  h,  tragus  and  tragic  muscle ;  i,  antitragus 
and  antitragic  muscle;  Jc,  the  edge  of  the  cartilage  which  is 
attached  by  fibrous  tissue  to  the  external  auditory  meatus  of  the 
temporal  bone ;  I,  tragus  from  behind ;  m,  transverse  muscle  cross- 
ing the  sulcus  at  the  back  of  the  antihelix;  n,  oblique  muscle 
crossing  sulcus  at  the  back  of  the  inferior  branch  of  the  anti- 
helix.  The  lobule  is  represented  in  dotted  oiitline. 


HEARING.  255 

189.  The  external  ear  consists  of  two  parts,  the  pinna  and 
the  canal.  The  pinna,  or  that  part  which  is  understood  when 
the  ear  is  spoken  of  as  a  feature,  presents  various  named 
inequalities  of  surface.  The  outer  border,  which  extends 
round  the  back,  and  curves  inwards  in  front,  is  called  the 
helix;  the  elevation  within  it.  forked  at  the  upper  part,  is 
the  anti  helix ;  while  the  hollow  at  the  bottom  of  which  the 
canal  is  placed  is  called  the  cup  or  concha.  The  little  eleva- 
tion in  front  of  the  canal  is  called  the  tragus,  the  similar 
elevation  behind  is  called  the  antitragus,  and  the  pendulous 
part  is  the  lobule.  The  pinna  consists  of  a  framework  of 
"cartilage  covered  with  integument;  but  at  the  lower  end  of 
the  helix,  the  cartilage  comes  to  a  point,  and  in  the  lobule 
there  is  nothing  but  a  mass  of  firm  adipose  tissue.  The 
lobule  is  sometimes  absent,  and  is  a  human  peculiarity,  the 
"beautifully  rounded  ears  of  monkeys  having  none.  The 
pinna,  in  many  animals,  is  obviously  useful  as  an  ear 
trumpet  for  gathering  sonorous  vibrations;  but  in  man  it  is 
of  comparatively  little  service,  although  it  is  provided  with 
muscles  which  give  it  a  slight  degree  of  movement.  One  of 
these  muscles  passes  from  the  parts  in  front,  another  from  above, 
and  a  third  from  the  mastoid  process  behind,  to  be  attached 
near  the  root  of  the  pinna;  and  they  are  named,,  respectively, 
the  attrahenSj  attollens,  and  retrahens  auriculam  muscles. 
There  are  likewise  various  still  smaller  muscles  which  pass 
from  one  part  of  the  pinna  to  another ;  thus,  one  bundle  on 
the  tragus,  and  another  on  the  antitragus,  pull  these  eminences 
very  slightly  downwards  and  apart;  two  slips  are  placed  on 
the  fore  part  of  the  helix,  and  the  antihelix  and  its  inferior 
branch  are  each  crossed  by  short  fibres  on  their  cranial  sur- 
face; but  the  only  interest  connected  with  these  muscles  is, 
that  they  represent  more  important  structures  in  the  lower 
animals.  So  also  there  is  a  little  tubercle  often  present  near 
the  upper  part  of  the  margin  of  the  helix,  which  is  interest- 
ing as  being  the  alleged  representative  of  the  tip  of  the  ear 
in  animals  which  have  the  pinna  pointed  (Darwin). 

The  canal  or  external  auditory  meatus  of  the  ear,  about 
an  inch  and  a  quarter  in  length,  is  partially  bounded  in  its 
outer  part  by  cartilage,  continuous  with  that  of  the  pinna; 
but?  more  deeply,  for  more  than  half  its  length,  has  osseous 


256 


ANIMAL    PHYSIOLOGY. 


walls,  which  belong  to  the  temporal  bone.  The  integument 
with  which  it  is  lined  secretes  cerumen  from  glands  of  a 
structure  similar  to  sweat  glands,  and  is  furnished,  towards 
the  superficial  extremity,  with  fine  hairs  inclined  outwards, 
so  as  to  offer  an  obstacle  to  the  entrance  of  particles  of  dust. 
Additional  protection  is  given  by  the  direction  of  the  canal, 
which  is  inclined  slightly  backwards  at  its  commencement, 
under  cover  of  the  tragus,  then  turns  a  little  forwards;  and 
also  in  the  outer  half  has  an  upward  slope,  which  is  suddenly 
changed  for  a  downward  inclination  in  the  deep  part. 


Fig.  127. — DIAGRAM  OP  THE  RIGHT  EAR.  a,  Osseous  part  of  the 
canal  of  the  external  ear;  b,  membrana  tympani  with  the  upper 
part  removed;  c,  malleus;  d,  incus;  e,  stapes  with  its  base  filling 
up  the  f enestra  ovalis  (the  f enestra  rotunda  is  seen  a  little  lower) ; 
/,  Eustachian  tube;  g,  tensor  tympani  muscle;  h,  stapedius 
muscle;  it  i,  portio  dura  of  the  seventh  nerre  divided;  1c,  mastoid 
cells;  Z,  m,  vestibular  and  cochlear  divisions  of  the  portio  mollis 
or  auditory  nerve;  n,  vestibule;  o,  cochlea. 

The  membrana  tympani  blocks  up  the  inner  end  of  the 
canal.  It  consists  of  a  fibrous  membrane  with  a  thin  cover- 
ing of  integument  on  the  outside,  and  of  mucous  membrane 


HEARING.  257 

within.  Its  fibrous  part  is  attached  to  the  bone  round  about, 
and  has  its  principal  fibres  radiating  from  the  lower  end  of 
a  process  of  an  ossicle  in  the  tympanum,  the  handle  of  the 
malleus,  which  descends  between  the  fibrous  and  mucous 
layers.  The  membrane  is  sloped  so  as  to  approach  nearer 
the  surface  at  its  upper  than  its  lower  edge;  and  it  is  slightly 
concave  on  its  outer  side,  being  pulled  inwards  at  the  point 
where  the  malleus  is  attached. 

190.  The  middle  ear,  called  also  the  cavity  of  the  tympanum 
or  drum,  is  a  space  of  greater  vertical  height  than  the  canal, 
and  still  more  extensive  from  before  backwards,  but  narrow 
transversely.  At  its  fore  part  is  the  opening  into  the  Eusia- 
chian  tube,  a  passage  about  an  inch  and  a  half  long,  leading 
into  the  pharynx  (p.  89).  This  tube  is  small  and  bounded 
with  bone  for  a  short  distance  at  its  tympanic  extremity; 
but  in  the  rest  of  its  extent  is  cartilaginous,  and  gradually 
widens.  Its  cartilaginous  wall  is  replaced  with  membrane 
at  its  lower  part,  and  is  so  related  to  the  levator  palati  muscle 
that,  in  the  act  of  swallowing,  that  muscle  momentarily 
closes  its  pharyngeal  extremity.  It  is  lined  with  ciliated 
epithelium,  as  is  also  the  tympanum.  It  allows  the  passage 
of  air  into  the  tympanum,  but  is  very  easily  blocked  up  by 
the  adhesion  of  its  walls,  near  its  fore  part;  and  if,  when 
this  occurs,  the  tympanum  have  either  too  much  or  too  little 
air  in  it,  the  effect  is  a  disagreeable  sensation  and  interfer- 
ence with  hearing,  liable  to  occur  after  violently  blowing  the 
nose,  and  producible  by  holding  the  nostrils,  and  making  a 
strong  expiration  with  the  mouth  shut,  so  as  to  force  air  into 
the  tympanum.  When  the  sensation  is  produced,  it  lasts 
for  a  variable  length  of  time,  according  to  the  extent  of 
contact  of  the  walls  of  the  Eustachian  orifice,  and  the 
viscidity  of  the  substance  which  causes  them  to  cohere;  and 
it  is  often  removed  by  the  instinctive  repetition  of  the  act 
of  swallowing,  and  stretching  the  neck  on  the  affected  side, 
so  as  to  make  a  greater  pull  on  the  floor  of  the  orifice,  as 
the  parts  fall  back  into  their  places  on  the  cessation  of  the 
spasm  of  deglutition,* 

*  It  is  only  fair  to  state  that  the  opinion  held  by  the  late  Mr. 
Toynbee,  that  the  Eustachian  tube  is  in  ordinary  circumstances  shut, 
and  is  momentarily  opened  in  swallowing,  is  held  by  many,  both  in 
H  3 


25S  ANIMAL    PHYSIOLOGY. 

Posteriorly,  the  tympanum  communicates  with  the  mastoid 
cells,  a  set  of  small  irregular  spaces  in  the  mastoid  part  of 
the  temporal  bone,  which  can  scarcely  be  supposed  to  be 
functionally  important,  as  they  vary  greatly  in  extent,  and 
have  little  development  in  young  persons. 

On  the  inner  side,  the  tympanum  is  bounded  by  the  petrous 
part  of  the  temporal  bone,  which  contains  the  internal  ear 
imbedded  in  it;  and  here  there  are  two  small  openings,  one 
above  the  other,  which  are  important  as  establishing  a  com- 
munication between  the  middle  and  internal  ear.  The  lower 
of  these  openings,  the  fenestra,  rotunda,  about  the  size  of  a 
pin  head,  is  blocked  up  with  a  membrane,  the  secondary 
membrana  tympani;  while  the  upper  opening,  the  fenestra 
ovalis,  somewhat  larger,  has  fitted  into  it  the  base  of  the 
stapes,  one  of  the  small  tympanic  ossicles. 

191.  The  tympanic  ossicles,  three  in  number,  the  malleus, 
incus,  and  stapes,  make  a  communication  between  the  niern- 
brana  tympani  and  the  internal  ear.  The  ossicles  termed 
malleus  and  incus,  from  a  fancied  resemblance  to  a  hammer 
and  anvil,  lie  one  in  front  of  the  other,  the  malleus  foremost. 
The  malleus  has  at  its  upper  part  a  head,  which  articulates 
behind  with  the  incus;  and  descending  from  the  head  is  the 
handle,  or  manubrium,  the  extremity  of  which  we  have 
already  seen  to  be  connected  with  the  membrana  tympani. 
Another  process,  procesxus  gracilis,  springs  from  below  the 
head  of  the  malleus,  and  passes  forwards  to  be  attached  in  a 
fissure  of  the  temporal  bone.  The  incus  articulates  by  its 
thickest  part  with  the  malleus,  and  sends  out  two  processes, 
one  of  which  projects  horizontally  backwards,  and  has  a 
ligamentous  attachment  in  front  of  the  mastoid  cells,  while 
the  other,  which  is  longer,  descends  vertically,  and  is  turned 

this  country  and  on  the  continent.  In  such  a  work  as  this  it  is  sufficient 
to  mention  two  objections  to  that  idea;  namely,  that'it  is  anatomically 
impossible,  and  that  I  have  myself,  as  elsewhere  recorded,  actually 
seen  through  a  perforation  in  the  palate,  the  extremity  of  the  Eusta- 
chian  tube  lying  open  when  the  throat  was  at  rest,  and  closed  in  the 
act  of  swallowing.  And  it  is  to  be  remembered  that  it  is  only  about 
the  extremity  of  the  tube  that  there  can  be  a  question;  since  no  one 
doubts  the  patency  of  the  osseous  part,  and  Kiidmger  has  demon- 
strated a  permanently  patent  canal  in  the  first  part  of  the  cartilagin- 
ous portion, 


HEARING. 


259 


inwards  at  its  extremity  to  articulate  with  the  third  ossicle, 
the  stapes.  The  stapes,  as  its  name  implies,  is  shaped  exactly 
like  a  stirrup;  it  lies  horizontally, 
its  head  articulating  with  the  incus, 
and  two  branches  extending  inwards 
to  its  base,  which  is  fixed  by  means 
of  a  ligament  which  surrounds  it  in 
the  fenestra  ovalis.  Thus,  it  will  be 
perceived,  the  tympanic  ossicles  are 

suspended  between  the  tip  of  the  Fig.  128.— TYMPANIC  Ossi- 
processus  gracilis  of  the  malleus  in 
front,  and  the  extremity  of  the  hori- 
zontal process  of  the  incus  behind, 
and  are  capable  of  a  swinging  motion 
round  the  line  joining  those  points, 
of  such  a  description  that  the  de- 
scending process  of  the  incus  swings 
outwards  or  inwards  with  the  handle 
of  the  malleus,  and  communicates 
its  movement  to  the  stapes. 

192.  This  is  precisely  the  principal 
movement  which  takes  place,  and  it 
is  accomplished  by  two  muscles. 
The  tensor  tympani  muscle  arises 
from  the  cartilaginous  wall  of  the  Eustachian  tube,  and 
entering  the  tympanum,  is  confined  by  a  ledge  of  bone 
to  the  inner  wall,  till  it  is  opposite  the  position  of  the 
malleus;  it  then  becomes  tendinous,  and  stretches  across 
the  cavity,  to  be  attached  near  the  base  of  the  handle 
of  that  bone,  pulling  it  inwards,  and  so  increasing  the  con- 
cavity of  the  membrana  tympani,  and  putting  that  membrane 
on  the  stretch.  An  antagonistic  muscle,  the  laxator  tympani, 
less  distinct,  arises  likewise  from  the  Eustachian  tube,  and 
is  attached  to  the  malleus  above  the  level  of  the  processus 
gracilis;  thus  pulling  the  head  of  the  bone  inwards,  and 
swinging  the  handle  outwards.  Now,  as  has  been  mentioned, 
the  internal  ear  is  filled  with  liquid,  which  is  incompressible; 
and  it  is  surrounded  with  unyielding  walls,  save  only  at  the 
two  fenestrse;  when,  therefore,  the  tensor  tympani  pulls  the 
handle  of  the  malleus  inwards,  so  as  to  make  the  membrana 


CLES  OF  JKIGHT  ±CAR.      a, 

Processus  gracilis  of  mal- 
leus; b,  posterior  process 
of  incus;  the  line  ab  is 
the  axis  of  rotation. 
When  c,  the  handle  of 
the  malleus,  with  fibres 
of  the  membrana  tym- 
pani radiating  from  its 
extremity,  is  pulled  in- 
wards by  e,  the  tendon 
of  the  tensor  tjonpani, 
the  incus  likewise  ro- 
tates arid  pushes  d,  the 
stapes,  in  at  the  fenestra 
ovalis;  /,  tendon  of  sta- 
pedius. 


260  ANIMAL   PHYSIOLOGY. 

tympani  tense,  and  the  incus,  partaking  of  the  movement  of 
the  malleus,  pushes  the  stapes  in  at  the  fenestra  ovalis,  it  is 
plain  that  the  whole  contents  of  the  internal  ear  are  sub- 
jected to  pressure,  and  that  the  secondary  membrana  tym- 
pani is  likewise  made  tense.  Thus  a  harmony  is  maintained 
between  the  condition  of  the  primary  membrana  tympani 
and  the  internal  ear. 

But  it  might  happen  that  the  nervous  structures  of  the 
ear  might  require  protection  from  violent  noise,  as  the  retina 
requires  protection  from  excessive  light,  and  gets  it  by  excit- 
ing a  reflex  action  which  contracts  the  pupil;  for  this  reason 
there  is  what  may  be  described  as  a  safety-valve  arrangement 
connected  with  the  stapes.  To  the  neck  of  the  stapes  a  tendon 
is  attached,  which  passes  back  through  a  foramen  in  the 
posterior  wall  of  the  tympanum,  and,  when  the  bone  is 
broken  open,  is  seen  to  be  continued  into  a  muscle  called  the 
stapedius  muscle.  This  muscle,  when  it  contracts,  pulls  the 
stapes  into  an  oblique  position  in  the  fenestra  rotunda,  and 
interferes  with  the  pressure  of  the  incus,  thus  relieving  the 
inner  ear  from  a  state  of  tension.  Judging  from  the  anatomy 
of  the  parts,  we  may  conclude  that  this  is  a  correct  view  of 
their  function,  and  fchat  the  stapedius  muscle  is  stimulated 
to  contract  by  a  reflex  action,  of  which  the  auditory  is  the 
incident  or  sensory  nerve. 

193.  The  internal  ear  is  imbedded  in  osseous  tissue;  and  the 
cavity  which  it  occupies,  the  osseous  labyrinth,  can  be  well 
studied  in  a  macerated  temporal  bone.  The  petrous  portion 
of  the  temporal  bone  is  a  long  three-sided  pyramid  of  hard 
consistence,  with  its  base  turned  towards  the  tympanum; 
and  in  its  posterior  wall,  which  looks  into  the  cranium,  there 
is  a  large  foramen,  the  meatus  auditorius  internus,  directed 
outwards.  When  this  meatus  is  looked  into,  it  is  seen  to 
terminate  very  soon  in  a  perforated  plate,  which  occupies  its 
lower  part,  and  leaves  an  opening  into  a  canal  above  it.  This 
canal,  termed  aquceductus  Fallopii,  gives  passage  to  the  portio 
dura  of  the  seventh  nerve,  otherwise  called  the  facial,  and 
has  nothing  to  do  with  the  organ  of  hearing;  but  the  per- 
forated or  cribriform  plate  transmits  the  portio  mollis  or  audi- 
tory nerve  to  the  internal  ear;  the  perforations  in  its  fore 
and  hinder  moiety  leading  respectively  into  the  anterior  and 


HEARING. 


261 


posterior  divisions  of  the  labyrinth.  The  posterior  part  of 
the  osseous  labyrinth  is  called  the  osseous  vestibule,  and 
contains  the  membranous  vestibule  floating  within  it;  the 
anterior  part  is  called  the  cochlea,  and  in  it  the  osseous  and 
membranous  parts  are  more  intimately  connected  one  with 
the  other.  However,  in  the  dry  bone,  the  parts  seen  are  the 
cavity  of  the  vestibule,  with  three  semicircular  canals  coming 
off  from  it  behind;  and,  in  front  of  the  vestibule,  the  cochlea, 
a  spiral  tube  coiled  on  itself  like  a  snail's  shell. 

The  cavity  of  the  vestibule  is  the  part  into  which  the  fenes- 
tra  ovalis  opens.  The  three  semicircular  canals  spring  from 
its  back  part;  they  are  about  a  twentieth  of  an  inch  in  width, 
and  each  makes  a  circuit  about  a  quarter  of  an  inch  in 
diameter.  One  of  them,  the  external,  is  placed  horizontally; 
the  other  two,  called  the  posterior  and  the  superior,  are 
somewhat  larger,  and  lie  in  planes  at  right  angles  one  to  the 
other;  the  posterior  being  close  to  the  hinder  surface  of  the 
bone,  and  the  superior  springing  from  a  part  which  is  common 
to  it  and  the  posterior  canal,  and  arching  outwards,  touching 
the  upper  surface  of  the  bone.  Each  semicircular  canal  has 
a  dilatation  or  ampulla  near  one  extremity. 


Fig.  129.  —  MEMBRANOUS  LABRYNTH,  diagrammatic  view,  a,  5,  cy 
Superior,  posterior,  and  external  semicircular  canals ;  d,  utricle; 
e,  saccule;  /,  canalis  reuniens;  g,  canalis  membranacea  cochleae. 

The  membranous  vestibule,  lodged  within  the  osseous,  is 
connected  with  it  merely  by  vessels  and  nerves.  It  presents 
two  cavities  separated  by  a  thin  partition :  the  anterior  is 
named  the  saccule,  and  connected,  as  we  shall  see,  with  the 
cochlea;  the  posterior,  which  is  larger,  is  called  the  utricle, 
and  gives  off  membranous  semicircular  canals,  which  lie  loose 


262 


ANIMAL   PHYSIOLOGY. 


in  the  osseous  canals  of  the  same  name,  and  likewise  exhibit 
each  an  ampulla.     The  fluid  in  which  all  these  membranous 
structures  float  is  called   the   peri- 
lymph,  and  that  which  they  contain 
is  called  the  endolympli.    The  nerves 
are  supplied  only  to  limited  portions 
of  the  membranous  vestibule,    one 
branch  passing  to   the  saccule,  an- 
other to  the  utricle,  and  one  to  a 
little  crescentic  elevation  projecting 
into  the  interior  of  each   ampulla. 
The    interior    of    the    membranous 
vestibule  is  lined  with  epithelium; 
and     exceedingly    slender     straight 
hairs  project  through  the  epithelium 
at  the  parts  supplied  with  nerves, 
and  are  probably  in  continuity  with 
the  nerve  fibres.     These  hairs  may 
be  supposed  to  vibrate  with  sono- 
rous vibrations;  and  to  make  such 
"  vibrations  stronger,  there  is  a  collec- 
tion of  minute  crystals  of  carbonate 
of  lime,  the  otoliths  or  otoconia,  in 
the  neighbourhood  of  the  termina- 
tion of  each  branch  of  nerve. 
Fig.    130.— NERVE-TERMI-       194.  The  cochlea  is,  in  its  early 
NATIONS  IN  AN  AMPULLA,  development,  an  outgrowth  from  the 
^MTullir'ner";  -stibule,    and    as    it    elongates    it 
fibres;  &,&,fusiform cells;  becomes  spirally  coiled,  taking  two 
c,    c,     auditory     hairs,   complete  turns  and  a  half,  tapering 
Rildinger,  ^0  ^  extremity,  and  acquiring,  as 

has  already  been  remarked,  the  appearance  of  a  snail's 
shell.  The  base  of  the  shell  is  at  the  perforated  plate  of 
the  inner  auditory  meatus,  the  apex  abuts'  against  the 
tympanic  end  of  the  Eustachian  tube,,  the  mouth  is  in 
connection  with  the  vestibule,  and  in  the  centre  of  the 
coils  of  the  tube  is  a  pillar  of  bone,  the  modiolus,  pierced 
with  canals  containing  the  cochlear  branches  of  nerve.  Around 
the  modiolus  is  a  spiral  ledge  of  bone,  the  lamina  spiralis,  pro« 
jecting  into  the  interior  of  the  tube;  and  continued  directly 


HEARING. 


203 


outwards  from  the  edge  of  this  is  a  fibrous  partition,  the 
basilar  membrcme,  dividing  the  tube  longitudinally  into  two 
parts,  and  attached  at  its  outer  edge  to  the  wall  of  the  tube 
by  muscular  fibres,  which  can  keep  it  tense.  Another  and 
much  more  delicate  partition,  the  membrane  of  Reissner, 
extends  upwards  and  outwards  from  the  lamina  spiralis  to 
the  outer  wall  of  the  tube;  and  thus  there  are  three  parallel 
passages  separated  from  one  another.  Of  these,  the  upper 
or  that  turned  towards  the  apex  of  the  cochlea,  is  called  the 
scala  vestibuli ,  and  commences  in  the  cavity  of  the  vestibule ; 
the  middle  passage,  placed  between  the  basilar  membrane 
and  membrane  of  Reissner,  is  called  the  canalis  membranacea, 
and  is  continuous  with  the  membranous  vestibule,  being  con- 
nected with  the  saccule  by  a  little  duct,  the  canalis  reuniens; 
while  the  lower  passage,  the  scala  tympani,  starts  from  the 
closed  fenestra  rotunda,  and  is  separated  from  the  vestibule 
by  the  basilar  membrane,  so  that  its  only  continuity  with 
that  cavity,  in  the  fresh  state,  is  by  a  small  opening  at  the 
apex  of  the  cochlea,  the  helicotrerna,  where  it  communicates 
with  the  scala  vestibuli  beyond  the  blind  extremity  of  the 


Fig.  131. — COCHLEA  OF  NEW-BORN  PIG,  section,  a,  Canalis  mem- 
braiiacea;  6,  scala  tympani;  c,  scala  vestibuli;  d,  basilar  mem- 
brane and  organ  of  Corti;  e,  membrane  of  Keissner;  /,  spiral 
ganglion.  Reichert. 


264 


ANIMAL   PHYSIOLOGY. 


canalis  membranacea.  It  will  be  understood  from  tins  that 
tlie  two  scalse  are  filled  with  perilymph,  while  the  canalis 
membranacea  is  lined  with  epithelium,  and  contains  endo- 
lymph. 


Fig.  132. — ORGAN  OF  CORTI,  diagrammatic  view,  a,  Basilar  mem- 
brane; b,  tough  structure  attached  to  the  edge  of  the  osseous 
lamina  spiralis,  termed  its  limbus,  and  presenting  a  toothed 
appearance;  c,  membrane  of  Keissner;  d,  d,  membrana  tectoria; 
e,  nerve  perforating  the  basilar  membrane ;  /,  /,  epithelial  cells ; 
g,  h,  inner  and  outer  groups  of  ciliated  or  hair-bearing  cells;  i,  kt 
inner  and  outer  rods  of  Corti ;  I,  membrana  velamentosa. 

In  the  interior  of  the  canalis  membranacea,  situated  on 
the  basilar  membrane,  is  the  sensitive  part  of  the  cochlea,  an 
exceedingly  complicated  structure  called  the  organ  of  Corti. 
This  organ  contains  numerous  sets  of  nucleated  cells,  some  of 
them  furnished  with  stiff  cilia  or  hairs,  and  it  is  permeated 
with  very  fine  ramifications  of  the  cochlear  nerve;  and  it  also 
contains  an  outer  and  inner  range  of  very  remarkable  strap- 
shaped  structures  of  comparatively  tough  consistence,  one 
range  leaning  against  the  other,  like  the  rafters  of  a  house. 
These  strap-shaped  structures  are  called  the  rods  of  Corti.  It 
is  curious  to  note  that,  while  the  osseous  cochlea  diminishes 
from  base  to  apex,  these  rods  increase  in  length  (Urban 
Pritchard),  and  the  basilar  membrane  on  which  they  lie 
increases  in  breadth  as  the  apex  is  approached  (Henle). 

195.  It  is  difficult  to  understand  the  exact  mode  of  action 
of  the  different  parts  of  the  internal  ear.  It  has  been  long 
generally  assumed  that  the  semicircular  canals  are  useful 
in  determining  the  directions  whence  sounds  proceed,  and 


HEARING.  265 

that  the  cochlea  is  a  kind  of  spiral  harmonicon,  vibrating  in 
different  parts  of  its  extent  in  unison  with  sounds  of  different 
pitch ;  and  these  appear  to  be  probable  suppositions ;  but 
neither  the  actions  of  the  semicircular  canals  nor  those  of  the 
cochlea  are  understood  in  detail. 

The  most  distinct  hearing  is,  beyond  question,  that  derived 
from  sounds  which  enter  by  the  external  ear.  We  turn  an 
ear  towards  a  sound  which  we  wish  to  hear  distinctly,  and 
hear  very  badly  when  the  ears  are  stopped.  The  membrana 
tympani  obviously  receives  principally  sounds  entering  by 
the  external  ear;  and  if  the  vibrations  of  that  membrane 
are  converted  into  a  swinging  movement  of  the  ossicles,  as 
experiments  seem  to  show,  and  are  thus  communicated  to 
the  labyrinth,  it  is  very  plain  that  vibrations  entering  by 
the  external  ear  can  be  of  no  use  in  enabling  the  semi- 
circular canals  to  determine  the  direction  from  which  a  sound 
is  coming.  It  would  appear  from  these  considerations  that 
direction,  except  in  so  far  as  it  is  determined  by  trying  in 
what  position  of  the  external  ear  a  sound  is  heard  loudest,  is 
appreciated  by  means  of  those  vibrations  which  pass  through 
the  bones  of  the  skull ;  and  as  bearing  on  such  a  supposition, 
it  may  be  mentioned  that  sounds  heard  when  the  ears  are 
thoroughly  stopped  are  sometimes  correctly  judged  as  regards 
their  direction,  and  that  fishes,  which  have  no  external  ears, 
or  only  minute  pores  to  represent  them,  have  very  large 
semicircular  canals.  It  is  also  possible  that  sounds  conveyed 
to  an  ampulla,  along  the  length  of  the  semicircular  canal  to 
which  it  belongs,  may  affect  it  more  than  others;  but  nothing 
certain  is  known  on  the  subject. 

It  must  not,  however,  be  forgotten,  that  we  are  often 
guided  to  the  direction  from  which  a  sound  comes  by  circum- 
stances which  have  nothing  to  do  with  the  ear,  such  as  expec- 
tation of  sound  from  a  particular  quarter,  or  the  direction  of 
the  eyes  of  onlookers.  So  also  the  distance  from  which  a 
sound  comes  is  judged  of  altogether  by  experience.  The  art 
of  the  ventriloquist  consists  simply  in  correctly  imitating  the 
effects  produced  by  sounds  at  different  distances,  and  in 
stimulating  the  imagination,  and  directing  the  attention,  so 
as  to  make  his  hearers  believe  that  a  sound  comes  from  a 
particular  quarter.  The  illusion  would  be  destroyed  if  the 


266  ANIMAL   PHYSIOLOGY. 

performer  were  to  show  any  movement  in  his  face  indicating 
speech;  but,  nevertheless,  his  voice  proceeds  from  his  larynx, 
and  the  words  are  formed  by  the  organs  of  speech,  and  the 
effect  is  produced  entirely  by  imitation  and  persuasion. 

The  precise  mode  of  action  of  the  cochlea  is  as  little  deter 
mined  as  that  of  the  semicircular  canals.  It  is  rudimentary 
in  birds,  and  in  its  spiral  form  is  peculiar  to  mammals.  It 
may  fairly  be  assumed  that  by  this  part  of  the  ear  we  become 
cognizant,  not  only  of  pitch,  but  likewise  of  the  quality  or 
timbre  of  sounds,  seeing  that  it  has  been  discovered  that 
timbre  depends  on  the  mixture,  with  a  principal  note,  of  a 
great  variety  of  others  in  consonance  with  it.  But  the  mode 
in  which  the  characters  of  sounds  are  preserved  unaltered  in 
their  passage  to  the  cochlea,  and  the  reason  why  the  rods  of 
Corti  get  longer  as  the  diameter  of  the  cochlea  gets  narrower, 
are  subjects  for  further  investigation.  It  cannot  be  doubted 
that  the  vibrations  of  the  hairs,  projecting  from  nucleated 
cells,  are  those  which  immediately  affect  the  auditory  nerve, 
and  that  they  are  produced  by  vibration  of  the  walls  of  the 
membranous  canal,  but  there  is  no  evidence  as  to  the  part 
played  by  the  rods  of  Corti.  It  ought  not  to  be  lightly 
assumed  that  they  strengthen  sound;  they  may  possibly  act 
as  dampers,  to  check  reverberation. 


CHAPTER  XVI. 


VOICE  AND   SPEECH. 

198.  Voice. — The  organ  of  voice  is  the  larynx,  a  modifica- 
tion of  the  upper  part  of  the  trachea,  consisting  of  a  frame- 
work of  cartilages,  lined  with 
mucous  membrane,  and  moved 
on  one  another  by  muscles. 

The  lowest  cartilage  of  the 
larynx  is  called  the  cricoid  car- 
tilage. It  forms  a  complete 
ring  above  the  first  cartilage  of 
the  trachea,  and,  while  narrow 
in  front,  rises  to  a  height  of 
more  than  half  an  inch 
behind. 

Above  the  cricoid,  and  par- 
tially embracing  it,  is  the 
thyroid  cartilage.  This  .carti- 
lage is  open  behind,  and,  at  the 
upper  border  in  front,  projects 
forwards,  making  the  promi- 
nence called  "  Adam's  apple." 
In  front,  its  lower  border  is  a  Fi  133._CABTILAGES  Or  THE 
little  above  the  cricoid,  and  the  LARYNX,  from  behind,  a, 
space  thus  left  is  filled  up  with 
elastic  tissue,  the  crico-thyroid 
membrane;  but  at  the  sides  its 
depth  increases,  and  it  sends 
upwards  and  downwards  two 
pairs  of  cornua;  the  inferior 
cornua  articulating  with  the 
sides  of  the  cricoid  cartilage,  so  as  to  furnish  a  centre  of 


Hyoid  bone ;  5,  lateral  thyro- 
hyoid  ligament ;  c,  thyroid 
cartilage;  d,  cricoid;  e,  ary- 
tenoid  cartilage,  surmounted 
by  cartilage  of  Santorini;  /, 
epiglottis  ;  g,  aperture  of 
glottis,  with  vocal  cord  on 
each  side. 


268 


ANIMAL   PHYSIOLOGY. 


rotation,  and  the  superior  being  united  by  ligaments  of  some 
length  to  the  hyoid  bone. 

Surmounting  the  back  part  of  the  cricoid  are  the  arytenoid 
cartilages,  two  bodies  shaped  like  three-sided  pyramids, 
articulated  at  their  bases  to  the  cricoid  cartilage,  and  curved 
backwards  at  their  apices,  so  as  to  give  the  mucous  mem- 
brane which  covers  them  an  appearance  in  the  middle  line 
like  the  spoilt  of  a  water-jug,  from  which  they  get  their 
name.  At  the  anterior  angle  of  the 
base,  they  are  prominent,  and  give 
attachment  to  elastic*  fibres  which 
pass  directly  forwards  to  be  attached 
to  the  thyroid  cartilage  close  to  the 
middle  line.  These  are  what  in 
1  strict  anatomical  language  are  known 
as  the  vocal  cords. 

But  by  the  term  vocal  cords  are 
most  frequently  understood,  not 
merely  the  few  fibres  mentioned, 
but  likewise  the  folds  of  mucous 
membrane  in  which  they  lie.  The 
mucous  membrane,  disposed  cylin- 
drically  in  the  interior  of  the  cricoid 
cartilage,  approaches  the  middle  line 
from  each  side  to  cover  the  elastic 
fibres  described;  and  is  then  ab- 
ruptly reflected  outwards,  forming 
on  each  side  a  hollow  called  the 
ventricle,  prolonged  into  a  little  sac- 
Fig.  131  -MESIAL  SEC-  d  in  front,  and  limited  above  by 
TION  OF  LARYNX,  «,  .,  J !  , ..  ,,  1  ,,  -  7  '. 

Hyoid  bone  ;  b  c  d,  a  semmmar  fold  called  the  jalse  vocal 
thyroid,  cricoid,  and  cord.  The  first-mentioned  folds,  or 
arytenoid  cartilages ;  e,  true  vocal  cords,  are  those  by  whose 
true  vocal  cord;/,  false  ^ration  the  voice  is 'produced;  in 
vocal  cord,  and  beneath  T  ,  •  ,  i 

it  the  ventricle  of  the  vocalization  they  are  approximated 
larynx;  g,  epiglottis;  A,  to  the  middle   line;    they  are  pro- 
tongue,  tected    with    squamous    epithelium 
from  the  force  of  the  air  which  whistles  past  them,  and  have 
their  position  and  tension  regulated  by  muscles.     The  space 
between  them  is  termed  the  glottis  or  rima  glottidis. 


269 


d. 


The  most  important  muscles  of  the  larynx  are:  (1)  a 
transverse  arytenoid  muscle,  uniting  the  posterior  surfaces 
of  the  arytenoid  cartilages,  and 
drawing  those  cartilages  together 
when  the  larynx  is  shut;  (2)  a  pair 
of  posterior  crico-arytenoid  muscles, 
passing  up  from  the  back  of  the 
cricoid  cartilage  to  the  arytenoid 
cartilages  at  their  outer  angles,  and 
rotating  the  vocal  cords  outwards, 
so  as  to  widen  the  glottis;  (3)  a 
pair  of  lateral  crico-arytenoid  muscles, 
passing  backwards  from  the  sides  of 
the  cricoid  to  the  outer  angles  of  the 
arytenoids,  and  rotating  the  vocal 
cords  inwards  to  the  middle  line ; 
(4)  a  pair  of  thyro-arytenoid  muscles 

lying  in  the  folds  of  the  vocal  cords,  Fi  *  135  _MuaoLE8  OF 
and  shortening  them  by  rotating  the  THE  LARYNX  ;  view  from 
thyroid  cartilage  upwards  on  the 
cricoid;  (5)  a  pair  of  crico-thyroid 
muscles,  seen  from  the  front,  and 
stretching  the  vocal  cords  by  rotat- 
ing the  thyroid  cartilage  downwards. 
197.  If  the  larynx  be  examined  by 
means  of  a  laryngoscope,  ih&t  is  to  say, 
a  mirror  placed  in  the  back  part  of 
the  throat,  so  as  to  throw  light  down 
on  the  larynx  and  reflect  its  image 
to  the  eye  of  the  observer,  it  will  be 
seen  that,  as  soon  as  vocalization 
commences,  the  vocal  cords  spring 
toward  the  middle  line,  leaving  only 
a  chink  between  them,  and  that  they 
as  quickly  recede  when  the  voice 
ceases.  Their  edges  are  turned  one 
toward  the  other  when  in  action ; 
but  at  other  times  they  are  everted. 
This  agrees  exactly  with  the  results 
got  by  tying  a  bit  of  india-rubber  or 


the  left  side  behind. 
Section  of  thyroid  car- 
tilage, the  left  side  of 
which  is  removed  with 
the  exception  of  5,  the 
part  articulating  with 
the  cricoid  cartilage ;  c, 
arytenoideus  muscle;  d, 
posterior  crico-arytenoi- 
deus  ;  e,  crico-thyroi- 
deus  ;  /,  crico-thyroid 
ligament;  g,  crico-ary- 
tenoideus  lateralis  ;  h, 
thyro-arytenoideus,  its 
upper  edge  correspond- 
ing with  the  edge  of  the 
vocal  cord;  i,  k,  thyro- 
and  aryteno  -  epiglotti- 
deus,  resting  on  the 
aryteno  -  epiglottideaii 
fold  of  mucous  mem- 
brane. A  portion  of  the 
mucous  membrane  is  re- 
moved between  h  and  k 
to  show  the  position  of 
the  glottis, 


270  ANIMAL   PHYSIOLOGY. 

other  membrane  to  the  end  of  a  tube,  and  holding  it  so  as 
to  leave  a  chink  between  two  tight  edges.  As  long  as  the 
edges  are  inclined  one  to  the  other,  or  are  parallel,  a  note  is 
produced  by  blowing  through  the  tube;  but  when  the  edges 
are  everted,  the  sound  ceases. 


Fig.  136. — LARYNX,  from  above;  laryngoscopic  views:  A,  in  deep 
respiration,  showing  the  trachea  down  to  its  bifurcation ;  B,  in 
uttering  a  high  pitched  note,  a,  Epiglottis;  &,  c,  swellings  cor- 
responding to  cartilaginous  nodules  of  Wrisberg  and  Saiitorini; 
d,  true  vocal  cord;  e,  false  vocal  cord.  After  Czermak. 

The  note  produced  by  vibrating  strings  and  laminae  is 
dependent  on  two  things,  namely,  the  length  of  the  string  or 
free  edge,  and  the  degree  of  tension.  Both  these  principles 
are  illustrated  in  the  human  larynx.  The  reason  why  the 
voices  of  women  and  children  are  higher  in  pitch  than  those 
of  adult  men  is,  that  in  women  and  children  the  larynx  is 
smaller  and  the  vocal  cords  are  shorter;  and  in  boys,  at  the 
age  when  the  voice  begins  to  grow  rough,  an  obvious  enlarge- 
ment of  the  larynx,  as  judged  by  the  prominence  of  the 
pomum  Adami,  may  be  observed.  But  the  different  notes 
of  any  one  voice  are  produced  by  varying  tension  of  the 
vocal  cords.  This  may  be  easily  proved  by  placing  a  finger 
over  the  space  occupied  by  the  crico-thyroid  membrane  and 
running  over  the  gamut,  when  the  thyroid  cartilage  will  be 
felt  gradually  coming  down  over  the  cricoid,  in  the  manner 
which  has  been  already  shown  to  stretch  the  vocal  cords. 
At  the  same  time,  the  whole  larynx  rises  more  nearly  to  a 
level  with  the  tongue,  and  hence  the  comparative  clearness 
of  the  higher  notes. 

The  various  cavities  above  the  level  of  the  vocal  cords, 
acting  as  resonating  chambers,  determine  the  timbre  of  the 


SPEECH.  271 

voice.  First  of  these  come  the  ventricles  of  tlie  larynx, 
while  above  are  the  pharynx,  nasal  fossae,  and  frontal, 
sphenoidal,  and  maxillary  sinuses  (p.  222);  and  among 
various  causes  which  combine  to  alter  the  tones  of  the  voice 
in  old  age,  may  be  mentioned  the  tendency  of  the  entrances  to 
these  sinuses  to  get  contracted  or  blocked  up;  for,  although 
the  dimensions  of  the  various  air-cavities  of  the  skull  get 
larger  in  advanced  life,  the  entrances  into  them  become 
smaller.  From  the  low  position  of  the  larynx,  in  the  utter- 
ance of  deep  notes,  overshadowed  as  it  then  is  by  the  root  of 
the  tongue,  the  voice  is  thrown  more  backwards  in  them,  and 
reverberates  more  in  the  various  sinuses. 

198.  Speech  is  to  be  carefully  distinguished  from  voice. 
Voice  without  speech  is  an.  inarticulate  sound;  speech 
without  voice  is  a  whisper.  Speech  is  accomplished  by  the 
modifying  action  of  various  organs  on  the  expiratory  currents 
of  air  passing  through  the  mouth.  The  tongue  is  by  no 
means  the  exclusive,  nor  even  the  principal  organ  of  speech. 
In  ordinary  language,  speech  is  referred  to  as  the  use  of  the 
tongue,  and  taste  as  the  use  of  the  palate;  and  even  the 
word  "language,"  etymologically,  means  action  of  the  tongue. 
Yet  the  palate  has  little  to  do  with  taste,  the  tongue  being 
the  only  part  in  which  that  sense  resides;  while  the  palate, 
the  lips,  the  fauces,  and  even  the  nose  are  organs  of  speech 
as  well  as  the  tongue,  and  persons  from  whom  the  tongue 
has  been  completely  removed  by  operation  continue  to  speak 
sufficiently  plainly  to  be  understood.  The  sounds  of  the 
various  vowels  are  made  by  varying  the  shape  of  the  aperture 
through  which  the  air  escapes  by  the  mouth;  the  consonants, 
on  the  other  hand,  are  sounded  by  placing  obstructions  of 
different  kinds,  and  in  different  places,  in  the  way  of  the 
current  of  air. 

In  the  following  table  an  attempt  is  made  to  arrange  all 
the  possible  consonant  sounds,  according  to  their  mechanism, 
as  labial,  palatal  or  dental,  and  guttural.  It  will  be  observed 
that  the  fore  parts  of  the  tongue  take  no  part  in  the  forma- 
tion of  any  labial  or  guttural  sound;  and  that  in  the  dental 
series  there  are  two  in  which  the  tongue  takes  no  part, 
namely,  sh  and  French  j,  the  only  two  sounds  in  which  the 
lower  teeth  take  part.  The  upper  teeth  take  part  in  two 


272 


ANIMAL   PHYSIOLOGY. 


sounds  ranged  with  the  labials,  namely,  /  and  v.  Corre- 
sponding hard  and  soft  sounds  are  placed  together  in  each 
series  in  the  table.  The  consonant  sounds  w  and  y  are 
omitted,  because  they  are  simply  a  rapidly  sounded  ou  and  ee. 


LABIAL. 

PALATAL  AND 
DENTAL. 

GUTTURAL. 

Obstruction  total  -     - 

'  P,  b 

t,  d 

k,  g  (gum) 

Incomplete  contact  of 
parts,    without    ob-  • 

f,  v 

(th(thirst)  I 
(  th  (then)  \ 

h,  the  burr 

struction  -     -     -     - 

s,       z 
sh,  j  (French) 

Vibratory  contact 

Inarticu- 
late sound 
like  pw 

r 

Inarticu- 
late gurgle 

Current  by  the  sides 

i 

of  an  obstruction    - 

i 

Current  reflected  into 

the    nose   from    an 

m 

11 

ns 

obstruction    -     -     - 

CHAPTER  XVII. 
REPRODUCTION  AND  DEVELOPMENT. 

199.  THE  simplest  method  of  multiplication  observed  in  any 
set  of  living  bodies  is  by  splitting  up  into  different  parts,  each 
of  which  becomes  a  distinct  individual.    This  is  called  multi- 
plication by  fissiparous  division,  and  is  principally  found  in 
the  very  simplest  forms,  such  as  unicellular  organisms.      It 
is  precisely  the  same  mode  of  multiplication,  occurring   in 
distinct  beings,   as  that  by  which  cartilage-corpuscles  and 
others  increase  in  number. 

When  small  portions,  or  buds,  are  separated  from  a  parent, 
the  mode  of  reproduction  is  said  to  be  of  a  gemmiparous 
description;  and  if  the  bud  should  happen  to  be  only  a  single 
nucleated  corpuscle  devoted  to  reproduction,  separated  from 
a  large  mass  of  such  corpuscles,  or  from  an  organism  how- 
ever complex,  yet  it  is  plain  that  it  may  be  none  the  less 
fairly  considered  as  a  bud  or  germ  from  the  whole  organism. 
Now,  that  is  precisely  what  an  ovum  essentially  is :  but  an 
ovum,  whether  vegetable  or  animal,  has  the  peculiarity  that 
it  will  not  develop  into  a  new  individual  unless  there  be 
incorporated  with  it  another  germ  of  dissimilar  kind,  though 
derived  from  the  same  species  of  organism;  and  herein  con- 
sists the  essence  of  sexual  reproduction.  The  germ  which 
appears  to  retain  its  individuality  before  and  after  fertilization 
is  the  ovum,  or  female  element ;  while  that  which  disappears 
by  being  incorporated  therewith  is  the  male  element. 

200.  In  the  present  state  of  science  no  explanation  can  be 
given  why  such  a  thing  as  sex  should  exist.     It  is  difficult 
to  say  how  far  down  in  the  organic  world  the  distinction  of 
sex  extends,  but  it  exists  in  organisms  of  exceedingly  simple 
character;  and  in  those  of  more  complex  descriptions,  although 
in   certain    instances  a  series  of  generations  are   produced 

U  s 


274  ANIMAL  PHYSIOLOGY. 

by  mere  gemmiparous  reproduction,   sexual  reproduction  is 
always  resorted  to  after  a  cycle  has  been  passed  through. 

The  modes  in  which  such  cycles  are  accomplished  are  very 
various.  In  many  instances,  both  in  cryptogamic  plants  and 
in  the  lower  forms  of  animals,  there  is  a  manifest  alternation 
of  generation.  Thus  the  spores  on  the  frond  of  a  fern  are 


Fig.  137. — ALTERNATE  GENERATION  OF  HYDROID  ZOOPHYTE  (Bougain- 
villia  ramosa).  A,  Zoophytic  form ;  at  medusoid.  B,  Liberated 
medusoid.  After  Allman. 

asexual  germs,  which  grow  up  to  form  a  lichen-like  plant, 
the  prothallus;  the  prothallus  bears  upon  it  male  and  femalo 
elements,  and  from  the  union  of  these  the  young  fern  takes 
its  rise.  So  also  among  animals,  in  many  hydroid  zoophytes 


REPRODUCTION   AND   DEVELOPMENT. 


275 


larger 


the  zoophytic  form,  which  is  the  more  largely  developed 
condition,  gives  origin  by  gemmation  to  medusoids  bearing 
male  and  female  elements,  and  from  the  fertilised  ova  of  these 
medusoids  new  colonies  of  zoophytes  take  origin.  There  are 
many  other  examples  of  alternation  of  generation  among 
animals;  but  the  interest  in  this  case  is  increased  by  the 
circumstance,  that  in  other  hydroid  zoophytes  the  buds  which 
bear  the  sexual  elements  remain  attached  as  mere  organs  of 
the  zoophyte,  never  attaining  to  an  independent  existence;  and 
thus  one  is  enabled  to  see  that  the  power  of  the  medusoid  to 
reproduce  the  zoophyte  is  but  a  modification  of  the  more  fre- 
quently exemplified  power  of  a  part  to  reproduce  a  whole , 
the  modification  being  that  the  reproductive  part  is  entirely 
severed  before  the  sexes  are  developed.  In  the 
Medusae,  or  jelly  fish,  a  similar 
alternation  takes  place,  only  the 
sexual  form  is  the  more  largely 
developed;  the  ovum  takes  root 
and  grows  into  a  body  which 
breaks  up  into  a  series  of  discs, 
each  of  which  is  developed  into 
a  medusa. 

In  the  Aphides,  or  plantlice, 
another  cycle  is  exhibited.     In 
the  interior  of  the  individuals 
derived     from    fertilized    ova, 
another  generation  is  developed  V         / 
from  unimpregiiated  germs ;  and    \  .  / 
the    insects    so  formed  become     J\A 
parents  of  others  in  like  manner;       j$ 
and  only  after   several  genera- 
tions are  perfectly  sexual  indi-  Fig>  533.  ^ALTERNATE  GENE- 


viduals  produced,    with  whom  - 
the  cycle  again  begins. 

In  bees  there  occurs  a  descrip- 
tion of  true  parthenogenesis. 
The  queen,  in  her  marriage 
flight  at  the  time  of  swarming, 
receives  the  male  element  into  a 
sac  provided  for  the  purpose,  and  afterwards  in  laying  her 


RATION  of  Aurelia  aurita.  A, 
Ciliated  ovum  become  ad- 
herent. B,  The  same  at  a 
later  stage  sending  out  pro- 
cesses. C,  Strobilus,  which 
breaks  up  into  separate  cups, 
each  of  which  becomes  an 
adult  Aurelia.  After  Sars. 


276 


ANIMAL   PHYSIOLOGY. 


eggs,  adds  or  withholds  a  little  of  the  contents  of  the  sac;  when 
this  is  withheld  the  egg  produces  a  male;  but  when  it  is 
added  a  female  is  produced,  which,  according  as  it  is  fed, 
becomes  a  worker  or  a  queen. 

In  vertebrata,  sexual  reproduction  is  the  only  kind  which 
occurs.  Yet  in  the  development  of  the  embryo  there  is  a 
set  of  phenomena  quite  homologous  with  alternate  genera- 
tion; for  the  whole  ovum  is  not  converted  into  an  embryo, 

but  only  a  part  of  it,  and,  there- 
fore, the  embryo  may  be  legi- 
timately considered  as  a  bud 
from  the  ovum;  in  which  case 
the  only  difference  between  ver- 
tebrate reproduction  and  that 
of  a  medusa  is,  that  in  the 
medusa  many  buds  are  derived 
from  an  ovum,  and  in  the  ver- 
tebrata there  is  only  one.*  As 
an  abnormal  variation,  the  single 
vertebrate  embryo  may  divide 
more  or  less  completely;  and 
this  is  the  origin  of  all  double 
such  as  two- 
headed  calves,  four-legged  hens, 
the  "  Siamese  twins,"  and  the  negress  sisters  exhibited  as  the 
"  two-headed  nightingale."  The  proof  of  this  is  found  in  the 
fact  that  embryos  in  early  stages  of  development  have  been 
seen  thus  partially  divided,  and  in  the  law  of  double  mon- 
strosities that  they  are  always  united  by  corresponding  parts. 
201.  Akin  to  the  power  of  reproducing  the  whole  individual 
is  the  power  of  reproducing  lost  parts;  and  the  law  may  be  laid 
down  that  the  less  advanced  the  development  of  the  species 
or  the  individual  the  greater  the  power  of  such  reproduction. 
So  great  is  this  power  in  some  invertebrate  animals  that  they 
may  be  multiplied  by  artificial  division,  each  moiety  retaining 

*  Viewing  the  vertebrate  embryo  as  a  bud  from  the  ovum,  a  com- 
parison by  no  means  vague  may  be  drawn  between  its  development 
and  that  of  a  tooth.  In  both  instances  there  is  an  elevation  which 
becomes  surrounded  by  a  fossa,  afterwards  converted  into  a  shut  sac, 
and  finally  the  shut  sac  is  burst. 


Fig.  139. — DOUBLE-HEADED  EM- 
BRYOS.   A,  Chick  (in  my  pos-  monstrosities, 
session).  B,  Perch  (Von  Baer). 


REPRODUCTION   AND   DEVELOPMENT. 


377 


the  power  of  completing  a  whole  form.  Even  animals  so 
complex  as  lobsters  have  notably  the  power  of  reproducing 
lost  limbs.  No  such  power  exists  in  vertebrate  animals  after 
birth,  with  the  exception  that  various  reptiles  and  fishes 
renew  their  tails  when  they  have  been  accidentally  lost;  sub- 
stituting, however,  calcified  chorda  dorsalis  for  the  lost 
vertebrae.  But  before  birth  lost  parts  may  be  reproduced, 
even  in  man,  to  a  surprising  extent.  Sometimes,  from  acci- 
dental causes,  a  limb  of  an  unborn  child  suffers  amputation 
by  means  of  strangula- 
tion by  a  band  of  lymph; 
and  in  such  cases  it  often 
occurs  that  fingers,  or  a 
whole  hand,  sprout  out 
from  the  stump  of  an 
arm,  although  they  fail 
to  reach  the  full  develop- 
ment. This  is  particu- 
larly interesting,  as  indi- 
cating the  latent  presence 
throughout  the  body  of 
the  reproductive  power 
which  is  exhibited  nor-  Fig.  140. — HUMAN  OVUM  within  Graafian 
rnally  and  in  perfection  ?esi?}Q-  a>  Germinal  vesicle  and  spot; 

b,  vitellus  or  yelk ;  c,  zona  pellucida; 

d,    discus    proligerus ;    e,    membrana 

granulosa ;  /,  vascular  wall  of  ovisac  ; 

g,    stroma  of    ovary;    h,    surface    of 

ovary. 

taining  hair  and  teeth  shows  the  presence  of  a  power  which 
is  normally  altogether  latent  till  the  addition  to  the  ovum 
of  an  element  containing  similar  qualities  locked  up  within 
it  causes  it  to  culminate  in  full  reproduction. 

202.  The  ovum  or  egg,  in  birds  and  various  other  animals, 
is  enlarged  by  the  incorporation  with  it  of  a  great  amount  of 
matter  which  does  not  undergo  fertilization,  useful  only  as 
material  for  the  nourishment  of  the  young  animal.  But  the 
mammalian  ovum  is  a  structure  microscopically  minute,  which 
is  only  discovered  by  scientific  observation. 

The  human  ovum  is  about  y^  of  an  inch  in  diameter. 
It  consists  of  a  transparent  envelope,  the  zona  pellucida, 


by  the  reproductive 
organs  alone.  So,  also, 
the  occasional  occurrence 
of  ovarian  tumours  con- 


278  ANIMAL  PHYSIOLOGY. 

surrounding  a  granular  yelk;  and,  in  the  interior  of  this, 
to  one  side,  is  a  clear  nucleus  called  the  germinal  vesicle, 
with  a  distinct  nucleolus,  the  germinal  spot.  The  ova  are 
developed  within  organs  called  ovaries,  which  are  placed  one 
on  each  side  in  the  lower  part  of  the  abdomen,  and  are  flat- 
tened oval  bodies,  about  an  inch  and  a  half  long,  invested 
with  peritoneum.  Each  is  attached  by  a  fibrous  cord  to  the 
upper  part  of  the  uterus. 


Pig.  141.-— UTERUS  AND  OVARIES  from  the  front,  a,  Vagina  with 
os  uteri  depending  into  it ;  6,  cavity  of  cervix  uteri,  with 
rugose  mucous  membrane ;  c,  cavity  of  f  undus  uteri,  exposed  in 
the  right  half ;  d,  round  ligament  of  uterus ;  e,  e,  Fallopian  tubes, 
the  right  one  laid  open;  /,  /,  ovaries;  g,  round  ligament  of  ovary; 
h,  parovarium, 

203.  The  uterus  or  womb  is  a  muscular  organ  placed,  in 
ordinary  circumstances,  within  the  pelvis.  It  is  a  pear-shaped 
body  about  three  inches  long,  flattened  from  before  back- 
wards, and  connected  with  the  abdominal  wall  in  each  groin 
by  means  of  a  round  ligament.  When  clipped  open  it  is  seen 
to  have  exceedingly  strong  walls;  its  lower  portion,  the 
cervix,  extending  for  about  an  inch  upwards  from  the  mouth 
or  inferior  opening  (os  externum),  surrounds  a  separate  cavity 
with  rugose  mucous  membrane,  divided  by  a  constriction 
(os  internum)  from  the  smoothly-lined  main  cavity,  destined 
for  the  reception  and  development  of  the  ovum.  This  main 
cavity,  contained  within  the  body  or  f  undus  of  the  uterus, 
although  destined  to  undergo  enormous  temporary  enlarge- 
ment in  pregnancy,  is  of  small  size  at  other  times,  with  its 


REPRODUCTION  AND  DEVELOPMENT. 


Fig.  142. — UTERUS  of  SHEEP. 
a,  a,  The  cornua. 


anterior  and  posterior  walls  in  contact,  and  its  three  borders, 
one  above  and  one  at  each  side,  all  bulging  inwards.  The 
upper  angles  are  distinguished  as  the  cornua,  being  parts 
which  in  many  of  the  lower  animals  are  greatly  elongated, 
so  as  to  render  the  uterus  completely  forked.  Into  each 
cornu  opens  a  long  narrow  duct,  the  Fallopian  tube,  which 
extends  outwards  in  front  of  the  ovary,  and  terminates  im- 
mediately beyond  it  in  an  expanded  and  fringed  opening,  the 
fimbriated  extremity,  which  leads  from  the  peritoneal  cavity, 
and  makes  a  communication 
between  it  and  the  outside  of 
the  body.  In  animals  in  which 
the  cornua  are  developed,  they 
are  the  parts  which  lodge  the 
young.  Thus,  in  a  sheep  with 
one  lamb,  the  lamb  occupies 
one  cornu;  if  there  be  twin 
lambs,  they  lie  one  in  each 
cornu;  and  in  animals  which 
have  a  litter  of  young,  the  em- 
bryos are  ranged  in  series  in  separate  membranes  along  the 
length  of  each  cornu. 

204.  The  ovaries  of  persons  who  have  died  in  the  prime  of 
life  present,  scattered  through  their  tough  fibrous  structure, 
and  more  or  less  distinctly  seen  from  the  surface,  a  variable 
number  of  clear  vesicles,  one  or  two  of  which  may  be  like 
very  large  beads  immediately  beneath  the  peritoneum.  These 
are  called  Graafian  vesicles;  each  of  them  contains  an  ovum, 
and  gradually  enlarging,  and  approaching  the  surface  as  it 
enlarges,  eventually  ruptures  and  discharges  its  ovum, 
covered  with  a  coating  of  granules,  the  discus  protigerus. 
The  ovum  is  caught  up  by  the  fimbriated  extremity  of  the 
Fallopian  tube,  which  would  appear  to  be  applied  to  the 
rupturing  vesicle  for  that  purpose. 

"While  only  a  limited  number  of  these  vesicles  are  visible 
with  the  naked  eye,  the  microscope  reveals  others  of  minute 
size  in  vast  multitudes,  which  have  been  estimated  at  more 
than  70,000  in  one  individual  (Henle).  The  ova,  more- 
over, make  their  appearance  prior  to  the  vesicles  which  subse- 
quently surround  them,  and  already  exist  in  large  numbers 


280 


ANIMAL   PHYSIOLOGY. 


before  birth.  The  germinal  vesicle  makes  its  appearance 
first,  then  the  rest  of  the  ovum;  the  ovum  subsequently 
becomes  imbedded  in  corpuscular  matter,  and  this  becomes 
separated  by  imbibition  of  fluid  into  two  strata,  one  of  which 
adheres  to  the  ovum,  and  is  the  discus  proligerus  alluded  to, 
while  the  other  adheres  to  the  ovisac,  and  is  named  membrana 
granulosa.  In  their  early  stages  of  development,  the  ova 
move  from  the  circumference  towards  the  attachment  of  the 
ovary,  the  smallest  ova  being  found  close  to  the  peritoneum. 
It  is  only  when  the  Graafian  vesicles  begin  to  fill  with 
fluid  that  they  push  their  way  in  the  direction  of  least 
pressure,  precisely  as  an  abscess 
would,  and  thus  approach  the  peri- 
toneum again,  at  the  same  time  that 
the  ovum  quits  the  centre  of  the 
vesicle,  and  adheres  to  the  outer 
wall.  The  originally  centripetal 
movement  reminds  us  that  although 
in  all  the  higher  vertebrata,  and 
many  fishes,  the  ova  escape  by  peri- 
toneal rupture,  yet  in  the  majority 
of  osseous  fishes  they  grow  in  fes- 
toons, directed  to  the  centre  of  a 
hollow  organ,  which  opens  by  a  duct, 
like  a  secreting  gland.  Also  the 
homologous  organ  in  the  male  is  a 
secreting  gland,  in  which  the  secre- 
tion travels  from  the  circumference 
towards  the  attachment;  and  a  rudi- 
mentary appearance  of  secreting 
Fig.  143.  —OVARY  AND  tubules,  in  which  the  ova  are  de- 

(Ortliraoforiscus      mola)       .  i  •       y 

laid  open.  malian  ovaries   by  some  observers. 

Thus  the  ovary  may  be  regarded  as 
an  imperfectly  developed  secreting  gland. 

At  periodic  intervals  of  a  month's  duration,  one  or  more 
Graafian  vesicles  ripen,  rupture,  and  discharge  their  contents. 
At  the  same  time,  the  mucous  membrane  of  the  uterus  shares 
in  the  vascular  excitement  which  the  ovaries  exhibit,  and 
this  passes  away  with  an  extravasation  of  blood  from  thg  sur- 


REPRODUCTION  AND  DEVELOPMENT.         281 

face.  Each  Graafian  vesicle  has  an  exceedingly  vascular  wall, 
embracing  the  proper  ovisac,  and  when  the  vesicle  bursts,  its 
vessels,  being  no  longer  supported,  give  way,  and  fill  the 
cavity  with  a  clot.  This  clot  and  the  sac  around  it  undergo 
various  changes;  the  clot  disappearing,  while  the  sac  becomes 
yellow,  and  increases  in  thickness  and  size,  particularly  when 
impregnation  has  taken  place.  The  structure  which  is  thus 
formed  is  called  a  corpus  luteum,  and  subsequently  dis^ 
appears,  leaving  nothing  but  a  cicatrix.  It  does  not  seem  to 
fulfil  any  function,  but  rather  to  be  a  growth  resulting  from 
the  vascular  excitement  of  the  structures  around. 

205.  In  the  male,  the  germ-preparing  glands,  corresponding 
to  the  ovaries  in  the  female,  are  the  testes.    They  differ,  how- 


Fig.  144.— DIAGRAM  OF  SEMINIFEROUS  TUBULES  AND  DUCTS,  a,  a, 
Tubuli  seminif eri ;  b,  b,  vasa  recta ;  c,  rete  testis ;  d,  vasa  effer- 
entia,  from  twelve  to  fifteen  in  number ;  6,  coni  vasculosi ;  /,  /, 
epididymis ;  g,  vas  aberrans ;  h,  vas  deferens. 

ever,  from  the  ovaries,  in  leaving,  before  birth,  their  original 
situation  within  the  body,  and  descending  to  a  position 
external  to  the  abdominal  cavity;  as  also  in  being  developed 


282  ANIMAL   PHYSIOLOGY. 

into  exceedingly  complex  tubular  glands,  with  their  ducts  in 
Structural  continuity  with  them.  The  technical  names  of 
the  different  parts  of  the  ducts  are  mentioned  in  the  explana- 
tion of  the  accompanying  diagram.  It  is  sufficient  to  note 
that  the  length  of  channel  through  which  the  secretion  has 
to  pass  is  without  parallel  in  the  rest  of  the  body ;  that  the 
secreting  tubules  are  upwards  of  two  feet  long,  and  that  the 
duct  called  the  epididymis  is  estimated  as  being  twenty  feet 
in  length,  and  is  ciliated.  The  advantage  gained  by  this 
complexity  is  not  known.  The  main  ducts  open  into  the 
urethra,  a  little  in  front  of  the  bladder,  where  that  passage 
is  surrounded  with  a  glandular  structure  called  the  prostate; 
and  in  the  middle  line,  close  to  the  two  ducts,  is  a  small 
pouch,  just  big  enough  to  admit  a  probe,  called  the  sinus 
pocularisj  interesting  as  being  the  structure  which,  in  the 
female,  is  developed  into  uterus  and.  vagina. 

The  male  germs,  or   essential  elements  of  the  secretion 
of  the  testes,  are  called  spermatozoa.     They  are  bodies  vary- 
ing from  -^  (j  to  ^o-  of  an  inch  in  length,  and  consist  of  a 
pear-shaped  body,  with  a  tail  extending 
out  from  the  broad  end.     The  tail  moves 
with  a  rapid  undulatory  movement,  which 
sends  the  spermatozoon  forwards,  body 
foremost.    The  spermatozoa  are  developed 
in  the  interior  of  cells,   the  protoplasm 
of  which  has  been  observed  adhering  to 
their  heads  in  the  young  state,  and  they 
may  very  probably  be  regarded  as    ele- 
ments    morphologically     equivalent     to 
Fig.    145.— SPEBMA-  nuclei.      Their  development  is  not  com- 
TOZOA.  pleted  till  they  leave  the  secreting  tubes. 

206.  Observations  on  the  lower  animals  leave  no  doubt 
that  fertilization  of  the  ovum  takes  place  by  the  entrance 
of  spermatozoa  into  the  interior  of  the  zona  pellucida,  after 
which,  both  spermatozoa  and  germinal  vesicle  are  melted 
down  in  the  yelk,  which  thereby  acquires  new  properties, 
and  divides  first  into  two  parts,  then  into  four,  and  so  on, 
each  part  dividing  always  into  two,  until  the  whole  yelk  is 
converted  by  this  process  of  cleavage  into  a  mass  of  nucleated 
corpuscles,  devoid  of  cell  walls,  from  a  certain  number  of 


BEPEODUCTION  AND  DEVELOPMENT. 


283 


which  the  future  animal  is  developed.  These  being  the  facts, 
the  student  may  see  that  impregnation  may  be  regarded  as 
the  fusion  of  two  mutually  attracted  units  of  life  into  one; 
and  that  it  is  possible  to  consider  the  ancestry  of  every 
nucleated  corpuscle  of  the  body  as  an  unbroken  chain, 
through  generations,  from  parents  to  children. 


Fig.  146.— CLEAVAGE  OF  THE  YELK.    The  dog,    Bischoff. 
In  its  passage  downwards  through  the  Fallopian  tube,  the 
ovum  undergoes  some  enlargement,  and  the  zona  pellucida 
receives  a  coating  of  albuminous  substance,  gradually  increas- 
ing in  thickness.     On  reaching  the  uterus,  the  envelope  of 
the  ovum,  henceforward  called  the  chorion,  proceeds  to  throw 
out  branching  processes  or  villi,  by  which  it  becomes  closely 
connnected  with  the  uterine  walls, 
and    receives    nourishment   from 
them;   and   in   these  villi   blood- 
vessels subsequently  appear,  con-  ^ 
iiected   with   the   embryonic    cir- 
culation.    The  mucous  membrane 
of    the   uterus,    rich   in    tubular 
glands,    and    ordinarily    covered 
with  ciliated  columnar  epithelium, 
begins,    even    before    the    ovum 
reaches  it,  to  become  thick  and 
spongy;  it  forms  a  growth  which, 
from  being  cast  oft'  at  the  birth  of 

the  child,  is  termed  decidua;  and  Eig/147.—  DIAGRAM  OF  DE- 
where  the  ovum  is  situated,  this 
rises  up  and  invests  it,  forming 
what  is  distinguished  as  the  de- 
cidua reflexa,  while  that  which 
lines  the  rest  of  the  uterine 


CTDUA.  a,  Decidua  vera; 
b,  decidua  reflexa;  c,  de- 
cidua serotina;  d,  ovular 
space,  with  villi  of  chorion 
round  about;  e,  mucus 
plugging  the  cervix. 


cavity  is  called  decidua  vera.     A  secretion  of  fluid   like- 


284  ANIMAL   PHYSIOLOGY. 

wise  takes  place,  and,  for  a  period,  two  separate  spaces 
exist  within  the  uterus,  namely,  that  of  the  general 
uterine  cavity,  and  that  which  contains  the  ovum.  It  is 
some  considerable  time  before  this  latter  space  grows  suffi- 
ciently for  the  decidua  vera  and  decidua  reflexa  to  come  into 
contact.  Ultimately  they  are  so  closely  blended  that  in  the 
later  months  of  gestation  the  decidua  reflexa  can  no  longer 
be  recognised. 

While  the  ovular  space  is  separated,  as  has  been  said,  from 
the  uterine  space  by  the  decidua  reflexa,  it  remains  from  the 
first  in  contact  with  the  uterine  wall  on  one  side;  and  the 
mucous  membrane  of  the  uterus  at  this  part  exhibits,  like 
the  rest,  an  exaggerated  growth,  sometimes  termed  decidua 
serotina,  destined  to  be  more  highly  developed  than  the 
decidua  vera,  and  becomes  the  medium  of  connection  between 
the  maternal  structures  and  the  child,  after  the  vessels  in 
the  villi  of  the  chorion  at  other  parts  have  disappeared.  The 
vessels  referred  to  in  the  villi  of  the  chorion  are  brought  to 
it  on  the  surface  of  a  vesicular  outgrowth  of  the  embryo, 
named  the  allantois  (p.  294);  and,  where  in,  contact  with  the 
decidua  serotina,  the  vessels  of  this  allantois  become  greatly 
developed,  as  well  as  those  of  the  uterine  mucous  membrane, 
and  a  structure  is  developed,  called  the  placenta  or  after- 
birth, by  means  of  which  the  formed  embryo  orfcetus  receives 
nourishment  from  the  mother  till  oirth. 

207.  The  Embryo. — We  have  seen  that  after  impregna- 
tion the  contents  of  the  ovum  are  converted,  by  the  disap- 
pearance of  the  spermatozoa  and  germinal  vesicle,  and  the 
cleavage  of  the  yelk,  into  a  mass  of  nucleated  corpuscles. 
This  happens  in  the  lower  part  of  the  Fallopian  tube,  and 
thereafter  the  interior  of  the  yelk  becomes  transparent,  and 
the  nucleated  corpuscles  are  aggregated  beneath  the  zona 
pellucida  in  the  form  of  a  hollow  sphere,  which  is  termed  the 
blastoderm  or  germinal  membrane.  Within  a  few  days  after 
the  ovum  has  reached  the  uterus,  a  clear  area,  with  an  opaque 
border,  makes  its  appearance  on  one  side  of  the  germinal 
membrane,  and  in  this  area  a  white  streak,  which  is  the  first 
appearance  of  the  embryo.  This  streak  consists  of  a  furrow 
with  elevated  margins,  the  primitive  groove;  and  beneath 
the  groove  a  rod-like  body  soon  appears,  the  chorda  dorsalis 


THE   EMBRYO. 


285 


or  notockord.  At  the  circumference  of  the  embryo  and 
beyond  it,  the  germinal  membrane  splits  up  into  two  layers, 
and  this  division  proceeds  completely  round  the  yelk;  but  if 
a  section  be  made  through  the  embryo,  three  layers,  much 
more  closely  connected,  are  seen;  the  innermost  of  which  is 
converted  into  the  epithelial  lining  of  the  alimentary  canal 
and  its  appendages,  while  the  middle  layer  forms  the  prin- 
cipal part  of  the  body,  and  the  outer  layer,  so  far  as  it  lies 
within  the  primitive  groove,  is  devoted  to  the  formation  of 
the  cerebro-spinal  axis,  and,  beyond  the  margin  of  the  groove, 
is  converted  into  the  cuticle  of  the  whole  body. 


Fig.  148.— PBIMITIVE  GROOVE  OF  Fig.  149.— SECTION  OF  MAMMA- 
RABBIT,  magnified  five  dia-  MAN  OVUM:  diagram,  a,  Outer 
meters,  a,  Area  ppaca ;  b,  area  layer  of  germinal  membrane ; 
pellucida;  c,  primitive  trace,  b,  inner  layer;  c,  primitive 
with  the  groove  in  the  middle.  groove,  and,  beneath  it,  section 
After  Bischoff.  of  chorda  dorsalis,  with  the  rest 

of  the  middle  layer  on  each 
side. ' 

208.  At  this  point  it  may  be  well  to  pause  for  a  moment, 
and  direct  the  student's  attention  to  some  of  the  peculiarities 
of  the  eggs  of  birds,  since  it  is  in  the  hen's  egg  that  by  far  the 
easiest  opportunity  is  obtained  for  studying  embryology  from 
nature.  The  hen's  egg  becomes  impregnated  in  the  upper 
part  of  the  oviduct,  and  the  cleavage  of  the  yolk  is  confined 
to  a  white  spot  at  one  side  called  the  cicatricula.  This 
takes  place  by  first  one  cleft  appearing,  then  four,  then 
others  between  them,  radiating  from  a  centre,  and  portions 
between  these  radiations  being  separated  irregularly,  and 
afterwards  subdividing;  but  the  yolk  beyond  the  cicatricula 


286 


ANIMAL   PHYSIOLOGY. 


•kikes  no  part  in  the  process.  In  succeeding  parts  of  tlio 
oviduct,  the  albumen  and  the  shell  are  deposited.  If  a  hen's 
egg  be  placed  on  its  side,  and  the  shell  be  broken  on  the  side 
which  happens  to  be  uppermost,  the  cicatricula  is  always 
found  on  the  corresponding  part  of  the  yelk.  The  reason  of 
this  is,  that  the  albumen  first  deposited  round  the  vitelline 
membrane  is  prolonged  in  two  twisted  strings,  chalazce, 


Fig.  150.— CLEAVAGE  OF  CICATRICULA  OF  HEN'S  EG  a.    After  Coste. 

towards  the  extremities  of  the  egg,  to  be  there  retained,  to  a 

certain  extent,  in  position.  By  these  chalaz83  the  yolk  is 

suspended,  and  being 
lighter  on  the  side  on 
which  the  cicatricula  is 
placed,  it  turns  that 
part  always  upwards 
away  from  the  damp 
ground,  and  towards  the 
warmth  of  the  hen's 
body.  In  consequence 
of  this  arrangement, 
there  is  nothing  easier 

'Fig.  151.—  HEN'S  EGG,  showing chalazoe,  than  to 'obtain  a  view 
and  embryo  of  three  days  incuba-  f  ^  earl  gt  of 

tion,  with  area  vasculosa  around  it.  ,  Jo, 

embryonic     growth     in 

the  chick,  from  the  appearance  of  the  primitive  groove 
onwards.  If  it  be  sought  to  hatch  the  eggs  artificially,  care 
must  he  taken  not  to  allow  their  temperature  to  vary  more 
than  a  few  degrees  above  or  below  102°F. 

&09.  The  primitive  groove  is  the  future  cerebro-spinal  canal, 


THE   EMBRYO. 


287 


and  is  converted  from  an  open  groove  to  a  closed  cylinder 
by  its  margins  growing  up  and  meeting  together  above  it.  In 
a  similar  way  its  contents  likewise  become  cylindrical;  the 
tube  thus  formed  being  converted  in  its  lower  part  into  the 
spinal  cord;  while  in  the  cerebral  part,  which  at  this  early 
stage  is  little  less  than  half  the 
length  of  the  whole  embryo,  it  is 
swollen  into  three  successive  vesi- 
cles. The  first  or  foremost  of 
these  cerebral  vesicles  is  that  from 
which  the  third  ventrical  of  the 
brain,  with  the  hemisphere-vesi- 
cles coming  off  from  it,  is  deve- 
loped; the  second  is  that  from 
which  the  aqueduct  of  Sylvius, 
with  the  corpora  quadrigemina  and 
other  parts  surrounding  it,  is 
formed;  and  the  third  is  the  part 
from  which  the  cerebellum,  pons 
Yarolii,  and  medulla  oblongata 
take  origin. 

The  chorda  dorsalis  or  noto- 
chord.  runs  down  the  centre  of 
the  middle  layer  of  the  embryo. 
It  is  a  purely  cellular  structure, 
and  continues  so  as  long  as  it 
exists,  but  in  most  animals  it  is 
not  permanent.  In  sturgeons, 
lampreys,  and  some  other  fishes,  it 
continues  through  life,  being  de- 
veloped into  a  thick  column  of 
large,  distinctly-walled  cells,  with 
a  thick  fibrous  sheath  round  about, 
which  serves  instead  of  a  chain  of 
bodies  of  vertebrae.  In  other  ver- 
tebrate animals,  the  bodies  of  the 
vertebrae  make  their  first  appear- 
ance round  the  sheath  of  the  chorda  dorsalis,  and  constrict 
that  structure  so  as  to  leave  a  bead-like  dilatation  in  the 
position  of  each  intervertebral  disc;  ultimately,  however,  in 


Fig.  152. — EMBRYO  CHICK, 
about  two  days  old;  under 
surface,  a,  b,  c,  First,  second, 
and  third  cerebral  vesicles; 

d,  primary  optic  vesicle; 

e,  rudiment   of   heart;   /, 
fold  at  which  the  cephalic 
plate   is    continuous    with 
the  cephalic  hood,  and  the 
yelk  sac  continuous  with 
the  cul-de-sac  from  which 
the  pharynx  is  formed ;  g, 
primordial    vertebrae  ;     h, 
unclosed  part  of  the  primi- 
tive groove. 


288 


ANIMAL    PHYSIOLOGY. 


the  higher  vertebrata  the  whole  chorda  'dorsalis  completely 
disappears,  even  the  intervertebral  discs  being  developments 
rather  of  its  sheath  than  of  its  proper  substance.  The  chorda 
dorsalis  has  been  traced  in  young  mammals  into  the  region 
of  the  sphenoid  bone,  where  it  ends  in  a  point  imbedded 
in  the  cartilage  of  the  base  of  the  skull. 

210.  On  each  side  of  the  chorda  dorsalis,  in  the  early 
embryo,  the  middle  layer,  except  in  the  head,  is  divided  into 
a  part  near  the  middle  line  called  the  dorsal  plate,  and  a  part 
beyond  termed  the  ventral  plate. 


Fig.  153. — TRANSVERSE  SECTION  OP  CHICK  of  two  days  incubation. 
a,  Spinal  cord;  &,  central  canal  of  cord;  c,  outer  layer  of  embryo, 
forming  the  cuticle ;  d,  primordial  vertebra ;  e,  chorda  dorsalis ; 
/,  inner  layer  of  embryo,  forming  intestinal  epithelium ;  g,  ven- 
tral plate  pushing  towards  the  middle  line ;  h,  *,  outer  and  inner 
division  of  the  same ;  k,  outer  layer  of  blastoderm,  from  which 
the  amnion  and  inner  part  of  the  chorion  are  formed ;  I,  inner 
layer  of  blastoderm  which  surrounds  the  yelk. 

The  dorsal  plates  soon  exhibit  a  distinct  segmentation, 
a  series  of  blocks  of  dense  tissue,  the  primordial  vertebrae, 
making  their  appearance  on  each  side  of  the  primordial 
groove,  beginning  behind  the  head,  and  increasing  in  num- 
bers backwards.  Each  of  these  so-called  primordial  vertebras 
contains  superficially  the  rudiment  of  a  segment  of  the 
muscular  wall  of  the  body,  and,  beneath  that,  the  commence- 
ment of  a  spinal  nerve,  which  afterwards  pushes  inwards  to 
join  the  spinal  cord;  also  between  the  successive  pairs  of 
spinal  nerves  appear  the  rudiments  of  the  skeleton,  namely, 
the  vertebral  and  costal  arches.  The  vertebral  and  costal 
parts  of  each  segment  are  indivisible  at  first,  but  afterwards, 
as  the  common  cartilaginous  mass  grows  outwards,  it  becomes 
divided,  in  the  thoracic  region,  into  vertebra,  rib,  costal  car- 
tilage, and  part  of  the  sternum,  and  folds  round  to  meet  its 
fellow  of  the  opposite  side. 


THE   EMBRYO.  289 

The  ventral  plates  early  split  into  a  deep  and  superficial 
part,  like  the  part  of  the  germinal  membrane  beyond  the 
embryo ;  but  at  their  inner  part,  instead  of  so  splitting,  they 
become  thicker  and  push  inwards,  so  that  the  plates  of  oppo- 
site sides  meet  together  below  the  chorda  dorsalis,  and  form 
a  mesial  plate,  in  which  appear  the  great  blood-vessels 
and  other  organs.  Among  these  organs  may  be  mentioned 
the  Wolffian  bodies  or  primordial  kidneys  (fig.  88),  which 
originally  occupy  the  whole  length  of  the  abdominal  cavity 
at  the  sides  of  the  vertebral  column,  and  are  closely  con- 
nected with  the  development  of  the  reproductive  organs; 
but  disappear  at  a  very  early  period  of  foetal  life,  and  are 
replaced  by  the  permanent  kidneys,  which  take  origin  between 
and  behind  them.  The  space  between  the  superficial  and 
deep  divisions  of  the  ventral  plates  is  the  great  serous  cavity 
of  the  trunk,  subsequently  subdivided  into  the  pericardial, 
pleural,  and  peritoneal  spaces.  The  superficial  division  is 
the  source  of  the  cutis  and  other  connective  tissues  of  the 
visceral  wall;  while  the  deep  division  adheres  to  the  inner 
layer  of  the  embryo,  and  completes  with  it  the  development 
of  the  alimentary  tube  (Remak). 

The  limbs  make  their  appearance  from  the  ventral  plates 
as  little  buds,  which  very  early  display  a  division  into  fingers, 
the  thumb  or  great  toe  of  each  lying  nearest  the  head;  and 
subsequently  the  elongation  of  the  arm  and  leg  takes  place. 

211.  In  the  head,  the  middle  layer  of  the  embryo  on  each 
side  and  in  front  of  the  chorda  dorsalis  is  called  the  cephalic 
plate.  We  have  seen  already  how  it  encloses  the  brain,  in 
the  same  fashion  as  the  dorsal  plates,  continuous  with  it, 
rise  up  round  the  spinal  cord.  Very  soon  the  brain  and 
parts  round  it  are  sharply  curved  down  towards  the  deep 
part  of  the  ovum,  the  margins  of  the  cephalic  plate  become 
folded  in,  and  a  deep  fossa  is  formed  round  it,  so  that  the 
head  and  neck  become  separated  off  from  the  ovum,  while  a 
hood  of  the  outer  layer  of  the  germinal  membrane  rises  up 
over  them.  In  like  manner  the  deep  layer  of  the  embryo 
within  the  head  becomes  converted  into  a  cul-de-sac,  sur- 
rounded with  the  continuation  forwards  of  the  mesial  plate. 

The  margins  of  the  middle  layer  in  the  head  and  neck  are 
thrown  into  five  pairs  of  processes  called  bran.chia.1  processes, 
14  » 


290 


ANIMAL  PHYSIOLOGY. 


separated  by  clefts ;  and  the  foremost  of  these,  uniting  with  its 
fellow  in  the  middle  line,  forms  the  lower  jaw,  while  the 
upper  part  of  the  cleft  behind  it  remains  as  the  opening  of  the 
ear,  and  the  other  clefts  entirely  disappear  in  the  higher  verte- 
brates, although  they  are  undoubtedly  the  same  clefts  as  those 
which  in  fishes  separate  the  gills  throughout  life.  In  front 
of  the  first  branchial  process,  the  eye  is  developed  in  a  cleft 
of  the  cephalic  plate,  and  separated  by  a  little  process  (lateral 
frontal)  from  the  nostril.  Between  the  nostrils,  the  mesial 
termination  of  the  cephalic  plate  is  prolonged  down  to 
form  the  middle  portion  of  the  upper  lip,  and  is  called  the 
middle  frontal  process.  The  cheeks  are  derived  from  the 
maxillary  lobe,  a  projection  sent 
forwards  from  the  region  at 
the  base  of  the  first  branchial  arch. 
The  mouth  makes  its  first  appear- 
ance as  a  depression  of  the  outer 
layer  of  the  embryo,  which,  at  a 
very  early  period,  touches  the  cul- 
de-sac  of  the  alimentary  tube;  and  a 
perforation  then  takes  place.  The 
roof  of  the  mouth,  after  this,  is 
formed  at  first  by  the  base  of  the 
skull,  as  it  continues  to  be  in  fishes 
throughout  life.  The  palate  is  a 

later    formation,   and  is   developed 

"magnlfiecTsix  diameters!  from  tne  maxillary  lobes,  which  each 
a,  Nostril,  and,  beneath  send  a  lamina  inwards  to  meet  its 
it,  the  middle  frontal  fellow  of  the  opposite  side  in  the 
process;  b  eye  and  be-  middle  line.  Sometimes  the  union 
neatli  it,  tne  lateral  fron-  r  .,  ,  ,  ,  ,  , 

tal  process;  c,  first  bran-  falls  to  te  completed,  and  the  result 
chial  process,  and,  be-  is  a  permanently  cleft  palate.  The 
tween  it  and  the  eye,  groove  over  the  upper  lip,  and  the 
the  maxillary  lobe;  d,  t  of  the  jaw  supporting  the  upper 
ear:  e,  ventricular  part  f  .  ,'t  T  •  ?  r 

of  the  heart;  /,   right  mcisor  teeth,  are  derived  from   the 
auricle ;  g,  fore  limb ;  h,  middle    frontal    process;    and  when 
Wolffian  body;  i,  place  this  fails  to  unite  with  one  or  both 
where  the  allantois  and  maxinary  io]bes,  the  result  is  a  single 
umbilical  vesicle   have         ,     ,  /  ,         \. 
been   torn   across  5   h  or  double  hare  lip. 
hind  limb,  21&  The  process  of  separation  of 


Fig.  154. — EMBRYO  LAMB, 


THE  EMBRYO,  291 

the  embryo  from  the  rest  of  the  ovum,  so  as  to  complete 
its  visceral  walls,  is  effected  by  the  folding-in  of  the 
layers  at  the  sides,  by  the  prolongation  backwards  of  the 
folding-in  which  has  been  spoken  of  as  occurring  in  the 
head,  and  by  a  similar  folding-forwards  at  the  pelvic  end 
of  the  body,  until  at  last  the  neck  of  communication 
of  the  embryo  with  the  rest  of  the  ovum  is  narrowed  to 
the  navel,  where  the  vessels  of  the  allantois  pass  in  and  out. 


Fig.  155.  —  DEVELOPMENT  or  THE  PALATE  IN  THE  EMBRYO  LAME, 
in  two  stages  more  advanced  than  that  exhibited  in  the  previous 
figure.  From  drawings  by  Professor  Dickson. 

By  this  means  both  the  cylinder  of  the  visceral  walls  and 
that  of  the  alimentary  tube  are  completed.  For  a  time  the 
remains  of  the  yelk  sac  continue  connected  with  the  alimen- 
tary tube,  which  projects  in  a  loop  at  the  navel  from  the 
completed  wall  of  the  abdomen,  and  they  go  by  the  name  of 
the  umbilical  vesicle,  while  the  pedicle  of  connection  is  called 
the  vitelline  duct ;  it  is  not,  however,  a  hollow  pedicle;  and 
even  in  those  classes  of  animals  which  have  large  yelk  sacs, 
the  contents  do  not  pass  into  the  alimentary  canal,  but  are 
absorbed  by  the  blood-vessels  round  it,  for  the  nourishment 
of  the  young  animal.  The  place  where  the  vitelline  duct  is 
attached  to  the  intestine,  is  near  the  lower  end  of  the  ileum. 
As  the  walls  of  the  embryo  become  folded  in,  the  outer 
layer  of  the  germinal  membrane,  which  is  gradually  expand 


292 


ANIMAL   PHYSIOLOGY. 


ing,  and  removed  from  the  inner  layer  or  yelk  sac  by  an 
increasing  amount  of  fluid,  rises  up  round  the  whole  embryo, 
as  it  has  previously  risen  round  the  head;  and  thus  the 
embryo  is  dipped  into  a  deepening  hollow,  and  at  last  the 
entrance  into  the  hollow  becomes  narrowed  and  obliterated 
above  the  dorsum  of  the  embryo,  and  the  embryo  is  com- 
pletely enveloped  in  a  sac,  which  is  filled  with  fluid,  and  is 
called  the  amnion.  The  part  of  the  outer  layer  of  the  ger- 
minal membrane  beyond  the  amnion  becomes  incorporated 
with  the  chorion.  The  amnion  continues,  till  birth,  as  a 
perfectly  transparent  membrane  filled  with  clear  liquor 
imnii,  in  which  the  foetus  lies. 


Fig.  156. — FCETAL  CONNECTIONS:  diagram,  a,  Sac  of  the  amnion; 
b,  yelk  sac ;  c,  allantois  becoming  developed  into  the  f cetal  part 
of  the  placenta. 

213.  We  have  still  to  consider  the  development  of  the 
Vascular  system.  The  first  traces  of  embryonic  blood-vessels 
seem  to  make  their  appearance  external  to  the  embryo,  in  the 
opaque  border  of  the  clear  area  which  surrounds  it.  Here 
there  is  a  circular  vena  terminalis,  and  a  development  of  blood- 
corpuscles.  A  network  of  vessels  springs  up  over  the  clear 
area,  and  the  blood  is  brought  to  the  embryonic  heart  by  two 
trunks,  which  enter  the  embryo  transversely,  one  on  each 
side,  the  omphalo-mesenteric  veins.  The  circular  vena  ter- 
minalis advances  on  the  inner  layer  of  the  germinal  mem- 
brane, until  the  network  of  vessels  of  which  it  forms  the 
limit,  the  ompkalo-mesenteric  system,  surrounds  the  whole 
yelk  sac.  This  is  the  earliest  system  of  blood-vessels  for  the 
nourishment  of  the  embryo;  and  in  the  eggs  of  oviparous 
animals,  which  are  comparatively  large,  it  is  of  great  import- 


THE  EMBRYO. 


293 


ance  in  absorbing  the  yelk  as  nourishment.  But  in  mammals, 
we  have  seen  that  the  ovum  is  minute,  and  grows  by  nour- 
ishment drawn  from  the  mother;  and  it  seems  probable  that 
in  them  the  omphalo-mesenteric  vessels  absorb  from  other 
sources  besides  the  contents  of  the  yelk  sac. 

214.  The  heart  is  at  first  a  straight  tube  which  runs  for- 
wards towards  the  head,  from  the  point  where  the  omphalo- 
mesenteric  veins  unite.  But  it  soon  elongates  so  much  that 
it  is  thrown  into  a  loop,  the  prominence  of  which  becomes 
converted  into  the  ventricular  portion  of  the  organ,  while 
the  lower  part  forms  the  auricular  portion,  and  the  upper  a 
common  arterial  trunk,  the  truncus  arteriosus.  Subsequently, 
the  truncus  arteriosus  is  divided  longitudinally  into  aorta 
and  pulmonary  artery,  and  the  auricles  and  ventricles  are 


Fig.  157. — PLAN  OF  THE  BRANCHIAL  ARCHES  IN  MAMMALS.  The 
portions  in  outline  are  obliterated,  a,  a,  a,  Aorta ;  6,  pulmon- 
ary artery;  c,  d,  right  and  left  pulmonary  arteries;  e,  truncus 
arteriosus,  obliterated  after  birth;  /,  innominate  artery;  g,  g,  g, 
subclaviaii  arteries  going  to  the  upper  limbs  ;  7i,  h,  vertebral 
arteries  ;  i,  common  carotid ;  k,  k,  internal  carotid ;  m,  external 
carotid  artery.  Rathke. 


291  ANIMAL   PHYSIOLOGY. 

each  lii  like  manner  divided  into  right  and  left  cavities.  But 
the  truncus  arteriosus  originally  passes  up  and  splits  into 
two  divisions,  from  which  come  off  five  pairs  of  branchial 
arches,  not,  however,  all  existing  at  one  time,  but  the  fore- 
most of  them  disappearing  as  the  hinder  arches  come  into 
view.  These  arches,  which  seem  to  correspond  with  the 
branchial  processes,  pass  partially  round  the  oesophagus,  and 
open  into  two  vessels  which  descend  side  by  side  in  front  of 
the  vertebral  column,  the  primordial  aortce.  The  two  aortse 
afterwards  become  fused  into  one,  which  continues  at  first  to 
arise  by  a  right  and  left  root,  and  permanently  does  so  in 
the  lower  classes  of  vertebrates;  but  in  birds  the  right  root 
alone  remains,  and  in  mammals  only  the  left  root,  while  the 
other  is  obliterated.  The  fourth  branchial  arch  of  the  left 
side  remains  in  mammals  as  the  arch  of  the  aorta,  and  the 
fifth  is  that  from  which  the  pulmonary  artery  is  developed. 

215.  At  the  period  of  the  closure  of  the  amnion,  while  the 
yelk  sac  is  still  widely  connected  with  the  intestine,  the  allan- 
toiSj  already  mentioned,  makes  its  appearance  in  front  of  the 
embryo,  at  the  lower  end  of  the  body  (fig.  156).  It  rapidly 
expands  into  a  large  vesicle  covered  with  blood-vessels,  and 
soon  becomes  constricted  in  the  middle.  The  vesicular  part 
at  the  base  remains  permanently  as  the  urinary  bladder;  the 
constriction  is  termed  the  urachus,  and  emerges  at  the  open- 
ing which  is  gradually  narrowed  to  form  the  navel;  and  the 
vesicle  beyond  continues  to  expand  till  it  reaches  the  chorion, 
then,  becomes  flattened  on  the  inner  surface  of  that  mem- 
brane, and  furnishes  blood-vessels  which  penetrate  into  all 
its  villi.  In  mammals,  a  great  development  of  blood-vessels 
takes  place  at  one  or  more  parts,  varying  in  different 
families;  and  at  these  parts  the  vessels  of  the  uterine  mucous 
membrane  likewise  expand  to  form,  in  conjunction  with  the 
allantoic  structure,  the  placenta,  which  consists  essentially 
of  aggregated  tufts  of  blood-vessels  belonging  to  the  foetus, 
projecting  into  sinuses  filled  with  maternal  blood,  whence 
they  suck  up  supplies  both  of  nourishment  and  oxygen. 
In  the  human  subject,  the  placenta  is  circular  and  limited  to 
an  area  which,  at  the  full  term,  is  only  about  7  or  8  inches 
in  diameter.  In  birds  and  reptiles  the  allaiitois  spreads  out 
as  in  mammals,  but  retains  its  membranous  and  hollo vr 


THE   EMBRtO  295 

construction,  and  has  principally,  if  not  entirely,  a  respiratory 
function.  In  amphibia  the  allantois  is  a  mere  urinary 
bladder,  and  the  period  of  allantoic  circulation  is  not  repre- 
sented. 

As  development  in  the  mamma*  proceeds,  the  placenta  is 
separated  by  a  greater  distance  from  the  embryo;  the  original 
hollow  of  the  urachus  and  placental  part  of  the  allantois 
disappears,  and  the  placenta  is  united  to  the  navel  by  an 
umbilical  cord,  consisting  of  vascular  trunks,  gelatinous  con- 
nective tissue,  and  an  investment  of  thickened  amnion.  The 
arteries  which  convey  the  blood  to  the  placenta  are  two  in 
number,  arising  within  the  pelvis,  and  pass  up  one  on  each 
side  of  the  urinary  bladder.  While  within  the  abdomen, 
they  are  termed  hypogasfoic,  and  in  the  cord  they  are  called 
umbilical  arteries.  The  veins  which  return  the  blood  from 
the  placenta  to  the  foetus  are  also  at  first  two  in  number, 
but  the  right  one  speedily  disappears,  so  that  in  the  cord 
there  are  only  three  vessels,  namely,  two  umbilical  arteries 
and  one  umbilical  vein.  These  are  twisted  spirally,  the 
arteries  having  the  appearance  of  winding  round  the  vein. 
The  umbilical  vein,  entering  at  the  navel,  passes  up  to  the 
under  surface  of  the  liver,  where  it  lies  in  the  longitudinal 
fissure  between  the  right  and  left  lobes  of  that  organ,  com- 
municates freely  with  the  portal  vein,  and  is  prolonged  back- 
wards, under  the  name  of  ductus  venosus,  to  join  the  inferior 
vena  cava. 

216.  When  it  is  considered  that  throughout  foetal  life  the 
lungs  are  of  no  service,  and  that  respiration  is  carried  on  by 
means  of  the  placenta,  it  will  be  at  once  perceived  that  the 
course  of  the  circulation  must  be  very  different  then  from 
what  it  is  afterwards,  and  that  a  sudden  change  must  take 
place  at  birth.  This  is  really  the  case;  and  in  connection  with 
the  course  of  the  circulation  there  are  some  foetal  peculiarities 
of  the  heart  and  pulmonary  artery.  In  the  fcetal  heart  the 
annulus  ovalis  of  the  right  auricle  is  an  open  foramen  ovale; 
and  the  pulmonary  artery,  after  giving  off  the  right  and  left 
pulmonary  arteries,  which  are  but  small  branches,  is  con- 
tinued straight  on  to  open  into  the  arch  of  the  aorta,  where 
that  artery  is  about  to  be  continued,  as  the  descending  aorta, 
down  through  the  thorax.  This  continuation  of  the  pul- 


295 


ANIMAL   PHYSIOLOGY. 


monary  artery  is  called  the  truncus  arteriosus;  and  it  may 
be  mentioned  that  fibrous  vestiges  of  this,  as  well  as  of  the 
ductus  venosus,  umbilical  vein,  and  hypogastric  arteries, 
remain  in  the  body  permanently. 

The   oxygenated  blood,  returning   to  the   foetus   by   the 
umbilical   vein,  passes  partly  into  the  vena  cava  inferior 


Fig.  158.— FCETAL  CIRCULATION.  The  course  of  i»he  blood  is  indicated 
by  arrows,  and  its  quality  by  shading,  a,  Umbilical  vein  bring- 
ing back  aerated  blood  from  the  placenta;  &,  ductus  venosns; 
c,  ductus  arteriosus;  d,  hypogastric  arteries;  e,  mesenteric 
vessels ;  /,  portal  vein,  and  above  it  the  hepatic  vein. 


FCETAL   CIRCULATION.  297 

directly,  and  partly  into  the  liver,  which  is,  throughout 
foetal  life,  exceedingly  large.  There  are,  therefore,  four  kinds 
of  blood  mixed  in  the  vena  cava  inferior  above  the  level 
of  the  liver,  namely,  vitiated  blood  from  the  lower  limbs, 
oxygenated  blood  from  the  placenta,  blood  which  has  passed 
through  both  placenta  and  liver,  and  venous  blood  from  the 
portal  system,  acted  on  by  the  liver,  but  not  oxygenated.  This 
mixture  of  blood,  entering  the  heart  behind  the  Eustachian 
valve,  is  directed  on  through  the  back  of  the  right  auricle 
and  the  foramen  ovale,  into  the  left  auricle,  and  so  into  the 
left  ventricle  and  arch  of  the  aorta,  to  supply  the  head,  neck, 
and  upper  limbs;  but  it  is  prevented  by  a  fold  in  the  aorta 
from  passing  down  into  the  descending  part.  Returning 
from  the  head  and  upper  limbs  by  the  superior  vena  cava, 
the  blood  enters  the  anterior  part  of  the  right  auricle,  and 
mixing  but  little  with  the  stream  crossing  through  the  back 
part  of  that  cavity,  passes  into  the  right  ventricle,  and  is 
propelled  through  the  pulmonary  artery,  ductus  arteriosus, 
and  descending  aorta,  partly  to  supply  the  lower  parts  of  the 
body,  but  in  larger  part  to  proceed  again  to  the  placenta  by 
the  hypogastric  arteries.  Thus,  curiously  enough,  there  is 
no  part  of  the  foetus  nourished  with  blood  the  whole  of  which 
has  been  purified.  The  whole  of  the  blood  returned  from  the 
lower  limbs  is  mixed  with  the  current  to  the  head  and  arms; 
and  the  trunk  and  lower  limbs  are  entirely  supplied  with 
blood  which  has  previously  circulated  in  the  head  and  arms 
and  been  filled  with  further  impurities. 

Immediately  on  birth,  the  circulation  through  the  placenta 
ceases,  the  impulse  to  breathe  begins,  and  with  the  first 
inspiration,  not  only  air  but  blood  is  drawn  into  the  expand- 
ing lungs;  the  right  and  left  pulmonary  arteries  are  filled, 
and  blood  ceases  to  pass  through  the  ductus  arteriosus;  the 
current  in  the  arch  of  the  aorta  is  continued  on  into  the 
lower  parts  of  the  body;  and  a  fold  of  membrane,  the  valve 
of  the  foramen  ovale,  which  springs  from  the  back  of  that 
opening,  and  projected  into  the  left  auricle,  now  occludes  the 
opening,  and  in  a  short  time  becomes  firmly  bound  down 
to  the  walls. 

217.  The  period  of  gestation  in  the  human  species  is  about 
two  hundred  and  seventy  days,  but  is  undoubtedly  liable  to  a 


298  ANIMAL    PHYSIOLOGY. 

certain  amount  of  variation.  Toward  the  end  of  that  time,  a 
great  relaxation  of  parts  takes  place,  even  the  ligaments 
which  bind  the  bones  of  the  pelvis  being  considerably  slack- 
ened; the  cervix  uteri  begins  to  dilate,  and  the  muscular 
walls  of  the  uterus  are  seized  with  recurrent  spasmodic 
contractions.  Usually  the  uterine  contractions  burst  the 
membranes  which  retain  the  liquor  amnii,  and  afterwards 
the  child  is  expelled,  its  head  first,  and  is  shortly  followed 
by  the  placenta  and  ruptured  membranes.  In  healthy  par- 
turition, the  contraction  of  the  uterus  is  sufficient  to  pre- 
vent much  bleeding  ensuing  from  the  tearing  across  of  the 
blood-vessels  which  united  the  placenta  with  its  substance. 
Forthwith,  the  uterus  begins  to  undergo  a  rapid  process  of 
involution.  Its  muscular  walls,  which  had  undergone  great 
enlargement,  both  by  increased  size  of  the  individual  fibres, 
and  continual  growth  of  others  from  connective-tissue  cor- 
puscles, become  daily  diminished  by  degeneration  and  dis- 
appearance of  the  exaggerated  fibres;  and  the  organ  returns 
to  its  original  proportions. 

218.  Within  a  day  or  two  after  the  birth  of  the  child,  the 
breasts  of  the  mother,  which  have  previously  been  enlarging, 
come  into  functional  activity,  and  there  is  a  copious  secretion 
of  milk. 

The  breasts,  or  mammce,  have  their  proper  secreting  struc- 
ture, at  other  times,  so  closely  connected  with  the  areolar 
tissue  in  which  it  is  imbedded  that  it  is  difficult  to  trace; 
but  during  lactation,  it  is  much  more  distinct.  From  about 
fifteen  to  twenty  separate  ducts  open  at  the  extremity  ol 
the  nipple;  each  of  these  is  dilated  into  a  small  reservoir 
between  one  and  two  inches  from  its  extremity,  and,  traced 
further  back,  is  found  to  branch  repeatedly,  till  the  radicles 
are  reached,  connected  with  the  ultimate  lobules,  which  con* 
sist  of  aggregations  of  rounded  secreting  saccule's.  ' 

Milk,  examined  microscopically,  is  a  clear  fluid,  with  oil 
globules  of  many  different  sizes  floating  in  it.  In  the  process 
of  churning,  the  oil  globules  of  milk  are  thrown  together  in 
a  solid  mass,  and  constitute  butter;  while  in  milk  which  is 
allowed  to  remain  at  rest,  so  that  the  larger  globules  rise  to 
the  top  without  running  together,  the  stratum  in  which  these 
accumulate  is  the  cream.  The  solid  constituents  of  milk  are 


GROWTH   AFTER   BIRTH.  299 

casein,  butter,  sugar  of  milk,  and  a  small  quantity  of  salts. 
As  compared  with,  the  milk  of  the  cow,  that  of  the  human 
species  has  a  smaller  proportion  of  solid  matter,  and  the 
solid  matter  contains  a  third  less  of  casein,  twice  as  much 
sugar,  and  not  half  the  quantity  of  salts  ;  and  on  that  account, 
in  feeding  infants  with  cows'  milk,  it  is  customary  to  mix  it 
with  sugar  and  water. 

The  first  milk  which  is  secreted  after  parturition  exercises 
a  purgative  influence  on  the  infant,  and  is  termed  colostrum; 
it  is  characterised  by  the  presence  of  corpuscles  consisting  of 
heaps  of  granules  gathered  together  into  balls.  After  pro- 
longed lactation  the  milk  secreted  becomes  poor  in  quality, 
and  the  continuance  of  suckling  becomes  hurtful,  both  to  the 
mother  and  to  the  child;  suckling  for  an  inordinate  length 
of  time  is,  therefore,  to  be  avoided.  The  best  food,  however, 
for  the  new  born  infant  is  the  mother's  milk;  and  no  more 
disgraceful  custom  can  well  be  imagined  than  a  healthy 
mother  neglecting  the  duty  of  suckling,  unless  it  be  that  of 
medical  practitioners  encouraging  such  an  impropriety. 

219.  Growth  after  Birth. — The  proportions  of  the  body  of 
the  new  born  infant  are  very  different  from  those  of  the 
adult.  The  umbilicus  is  about  the  middle,  the  lower  limbs 
and  the  chest  are  small,  and  the  head,  as  compared  with 
the  rest  of  the  body,  is  much  larger  than  afterwards.  In 
representing  the  adult  figure,  the  rule  recognised  in  art  is  to 
allow  eight  timos  the  perpendicular  length  of  the  head  for 
the  height  of  the  whole  body,  thus :  from  the  crown  to  the 
chin,  from  the  chin  to  the  level  of  the  nipples,  from  that 
level  to  the  navel,  and  from  the  navel  to  the  pubis,  each  one 
head ;  and  from  the  pubis  to  the  lower  part  of  the  knee,  and 
thence  to  the  sole,  each  two  heads.  The  position  of  the 
umbilicus  is,  as  will  be  perceived  from  this,  considerably 
higher  in  the  adult  than  the  child. 

The  pelvis  in  the  child  is  remarkably  small,  and  is  so 
situated  at  birth  that  the  sacrum  lies  pretty  nearly  in  a 
continuation  of  the  line  of  the  vertebral  column;  but  when 
the  child  begins  to  walk,  the  stretching  out  of  the  thighs  is 
effected  much  more  by  bending  the  sacrum  back  than  by 
movement  at  the  hip  joints,  and  the  brim  of  the  true  pelvis 
is  thrown  into  a  nearly  vertical  plane;  whereas  in  the  adult 


300  ANIMAL   PHYSIOLOGY, 

it  is  at  an  angle  of  about  30°  with  the  vertical  plane.  This 
circumstance,  in  conjunction  with  the  shortness  of  the  chest, 
is  the  cause  of  the  prominent  appearance  of  the  abdomen  in 
children. 

The  growth  of  the  head  deserves  especial  attention.  The 
maximum  proportion  of  the  cranium  to  the  rest  of  the  body 
is  found  in  the  early  state  of  the  embryo,  and  it  continues  to 
diminish  till  maturity  is  reached.  At  birth,  the  parietal 
region  is  particularly  prominent,  while  both  frontal  and 
occipital  regions  are  comparatively  small.  During  child- 
hood, the  forehead  becomes  sufficiently  developed  to  produce 
that  projection  forwards  at  the  level  of  the  frontal  eminences, 
which  gives  a  characteristic  appearance  to  the  child's  head ; 
but  these  eminences  continue  to  ascend  to  a  higher  level 
above  the  eyes,  and  to  separate  one  from  the  other  for  a 
number  of  years;  and  the  projection  forwards  of  the  lower 
part  of  the  forehead,  including  the  frontal  sinus,  is  not 
completed  till  after  puberty.  A  comparatively  small  de- 
velopment both  of  the  frontal  and  the  occipital  region  is 
characteristic  of  the  female  sex. 

The  face  is  exceedingly  small  in  children.  It  undergoes 
development  in  connection  with  both  first  and  second  denti- 
tion; but  it  does  not  reach  its  full  proportions  till  after 
puberty,  and  remains  permanently  smaller  in  females  than 
in  males. 

The  eyeballs  and  the  internal  and  middle  ear  are  nearly  as 
large  at  birth  as  in  the  adult. 

Probably  the  period  of  maximum  vital  energy  is  best 
indicated  by  the  vital  capacity  of  the  chest,  which  is  greatest 
about  the  age  of  thirty.  As  age  advances,  the  diminished 
rapidity  of  the  pulse,  and  greater  difficulty  in  repair  of 
injuries,  indicate  the  decline  of  nutritive  activity,  and  at 
last,  as  years  accumulate,  this  becomes  insufficient  to  sup- 
port the  processes  necessary  for  life. 

220.  Death. — The  precise  causes  of  diminished  vitality  in 
old  age  are  not  known;  but  it  is  worthy  of  note  that  the 
corpuscular  elements  of  the  tissues  have  each,  apparently,  a 
life  of  only  limited  duration,  and  that  they  diminish  in  number 
as  the  individual  becomes  older,  being  excessively  abundant 
before  birth,  and  most  sparingly  distributed  in  old  age.  Thus 


DEATH.  301 

the  limit  to  the  duration  of  vitality,  which  necessitates  a  system 
of  reproduction  for  the  continuance  of  the  species,  is  not  con- 
fined to  the  organism  as  a  whole,  but  is  found  in  the  vital 
elements  of  which  it  is  composed. 

During  health,  we  have  seen  that  though  the  microscopic 
constituents  of  the  body  are  continually  changing,  and  the 
vital  elements  dying  and  replaced  by  others,  yet  so  perfect 
are  the  arrangements  for  the  removal  of  debris,  that  no 
accumulation  of  effete  matter  takes  place.  The  baneful 
effects  of  even  minute  quantities  of  putridity  in  contact  with 
living  textures  are  now  well  known  to  surgeons,  and  the 
knowledge  furnishes  the  basis  for  the  proper  treatment  of 
sores.  But  sometimes  it  happens  that  in  consequence  of 
irritation,  either  from  without  or  within  the  body,  the 
nutritive  actions  in  the  textural  elements  are  deranged,  the 
living  particles  attract  different  constituents  from  the  blood 
than  they  are  wont  to  do,  act  differently  on  them,  and 
undergo  rapid  proliferation  and  disintegration.  Examples 
of  such  a  process  are  found  in  ulceration  and  suppuration, 
in  which,  although  the  pus  thrown  out  consists  of  living 
corpuscles,  and  is  the  result  of  vital  action,  yet  there  is 
increased  mortality  of  textural  elements  and  disruption  of 
texture.  Sometimes  it  happens  that  a  whole  mass  of  texture 
is  deprived  of  vitality  by  over-irritation,  chemical  alteration, 
withdrawal  of  the  supplies  of  nourishment,  or  other  inter- 
ference with  its  nutrition.  Such  a  mass  undergoes  decom- 
position all  the  more  rapidly  that  it  is  in  contact  with  the 
warm  body;  it  is  called  a  slough  or  sphacelus,  and  when  the 
texture  which  has  died  is  bone,  the  death  is  termed  necrosis 
and  the  dead  part  a  sequestrum. 

221.  As  it  is  with  portions  of  the  body,  so  also  with  the 
whole  being;  death,  considered  physiologically,  is  the  perma- 
nent cessation  of  nutrition.  The  cessation  of  consciousness 
is  not  death;  life  may  continue  when  consciousness  is  gone; 
and  the  relation  of  consciousness  to  the  body  is  that  it  is 
dependent  on  the  nutrition  of  the  brain. 

Death  is  sometimes  spoken  of  as  beginning  either  at  the 
heart,  the  lungs,  or  the  brain,  which  constitute  the  tripod  of 
life;  but  the  cessation  of  circulation,  leading  to  the  with- 
drawal of  fit  nutriment  from  all  the  textures,  the  nervous 


302  ANIMAL   PHYSIOLOGY. 

system  included,  and  their  saturation  with  debris,  may  claim 
to  be  the  immediate  cause  of  death  in  all  instances. 

If  it  be  suggested  that  it  would  be  better  to  define  death 
as  the  separation  of  the  spirit  from  the  body,  the  answer  is 
simply  that  the  presence  or  absence  of  the  spirit  does  not 
immediately  affect,  as  far  as  can  be  seen,  the  vitality  of  the 
organism;  and  that  physiology  has  no  means  to  ascertain 
the  moment  of  the  spirit's  withdrawal.  Hare,  fortunately 
exceedingly  rare,  cases  have  occurred  of  persons  apparently 
dead  returning  to  life  after  days  of  pulseless  trance.  In. 
these  instances,  the  body  remained,  throughout  the  trance,  fit 
to  resume  its  functions.  This  it  cannot  do  where  there  is 
decomposition.  Thus,  in  the  case  of  muscular  fibre,  we  have 
seen  that  even  a  very  slight  chemical  change  is  incompatible 
with  vitality,  as  tested  by  electric  instruments.  Decom- 
position is  the  infallible  evidence  of  death. 


GLOSSABY. 


Ab  omasum,  ab  from,  and  omasum; 
leading  from  the  omasum,  the 
fourth  stomach  of  a  ruminant. 

Absorption,  ab  from,  and  sorbeo  I 
suck ;  the  taking  up  of  anything 
into  the  system,  whether  from 
without  or  from  the  tissues. 

Acetabulum,  a  cup  for  holding 
vinegar,  the^  cavity  in  the  inno- 
minate bone  for  the  hip  joint. 

Aeromion,  &Kpov  a  summit,  and 
OVXGS  a  shoulder ;  the  process  of 
the  scapula  which  forms  the 
summit  of  the  shoulder. 

Adipose,  adeps  fat;  fatty:  thus, 
adipose  tissue,  the  tissue  in 
which  oil  is  stored. 

Agminated,  agmen  an  army  or 
company ;  agminated  glands, 
named  from  being  disposed  in 
groups. 

Albino,  albus  white;  an  Italian 
word  for  a  person  destitute  of 
pigment  in  the  hair  and  eyes. 

Albuminoid,  albumen  white  of  egg, 
and  eloos  form ;  belonging  to  the 
same  chemical  group  as  albu- 
men. 

Alimentation,  alimentum  nourish- 
ment ;  the  taking  of  nourish- 
ment into  the  system. 

Allantois,  aAAas  a  sausage  ;  a  hol- 
low outgrowth  from  the  embryo, 
from  which  are  developed  the 
urinary  bladder  and  the  pla- 
centa. 

Amnion,  a/nviov  from  &/ULVOS  a 
lamb ;  the  inner  membrane 
round  the  fcetus. 

Amoeboid,  d/m'jSw  I  change,  and 
tldos  formj  like  an  amoeba,  a 


genus    of    exceedingly    simple 
animals  of  changeable  form. 
Ampulla,    anything    blown    out 

into  a  swelling. 

Amyloid,  a/u.v\ov  starch,  and  6i<5o? 
form ;  like  starch.  Amyloid 
substance  of  the  liver  is  closely 
akin  to  starch ;  but  the  sub- 
stance peculiar  to  amyloid  or 
waxy  degeneration  of  the  liver, 
kidneys,  muscles,  &c.,  is  nitro- 
genous, and  has  no  chemical 
affinity  to  starch. 
Amylolytic,  a/mv\ov  starch,  and 
Xuw  I  loose  ;  having  the  power 
of  converting  starch  into  dextrin 
and  sugar. 

Anatomy,  &va  up,  and  re/mvu)  I 
cut ;  etymologically  means  dis- 
section ;  the  science  founded  on 
dissection,  and-treating  of  struc- 
ture in  organisms. 
Annulus  ovalis,  oval  ring ;  a  struc- 
ture in  the  right  auricle  of  the 
heart. 

Anorthoscope,  avop66<a  I  set 
straight  again,  and  o-/co7rew  I 
behold ;  an  instrument  so  con- 
structed that  distorted  images 
drawn  on  cards  prepared  for 
the  purpose,  on  being  placed  in. 
it  and  whirled  rapidly  round, 
are  seen  restored  to  their  just 
proportions. 

Antinelix,  avrL  opposite,  and  helix 
the  bifurcated  elevation  within 
the  circle  of  the  outer  ear, 
Antitragus,  &vrl  opposite,  and 
traffus  the  hinder  of  the  two 
little  elevations  opposite  the 
opening  of  the  ear, 


304 


GLOSSARY. 


Antrum,  a  cave. 

Aorta,  aeipa  I  raise  up;  in  the 
passive,  to  arise  ;  the  artery  of 
origin. 

Aphasia,  d  privative,  and  <£a<ris 
speech ;  loss  of  the  mental 
faculty  of  speech,  as  distin- 
guished from  paralysis  of  the 
organs  of  speech. 

Aponeurosis,  UTTO  from,  and  vzvpov 
a  sinew  ;  white  fibrous  tissue  of 
tendinous  consistence  spread 
out  in  a  sheet. 

Arachnoid,  apayvr]  a  spider's  web, 
and  eloos  form ;  the  serous  mem- 
brane which  surrounds  the  brain 
and  spinal  cord. 

Areolar,  areola  a  little  space ; 
areolar  tissue,  a  form  of  white 
fibrous  tissue  named  from  the 
spaces  between  the  felted  fibres. 

Artery,  arteria  an  air  vessel,  as 
arteria  aspera  the  wind-pipe  ;  a 
vessel  carrying  blood  away  from 
the  heart,  supposed  by  the 
ancients  to  contain  animal 
spirits. 

Arteriole,  a  little  artery. 

Arthrodia,  apQpov  a  joint;  an 
articulation  permitting  little 
movement. 

Arytenoid,  apvraiva  a  ewer  or 
ladle,  and  £i<5os  form ;  the  name 
given  to  two  cartilages  of  the 
larynx,  on  account  of  the  spout 
formed  between  them. 

Asphyxia,  a  privative,  and  cr$u£«> 
I  throb ;  cessation  of  pulse 
caused  by  cessation  of  breath- 
ing, choking,  suffocation. 

Assimilation,  slmilis  like;  the 
power  by  which  living  bodies 
convert  matters  from  without 
into  their  own  substance. 

Astragalus,  ao-rpdyaXos  the  bone 
by  which  the  foot  articulates 
with  the  leg.  That  of  the  sheep 
was  used  by  the  ancients  as  a 
kind  of  dice. 

Atlas,  the  god  who  bore  up  the 
pillars  of  heaven  ;  the  first  cer- 
vical vertebra. 

Auricle,  auricula  the  outer  ear. 
The  auricles  of  the  heart  are 
named  from  a  fancied  resemb- 


lance of  the  auricular  appen- 
dages to  dogs'  ears. 

Automatic,  auT-o/xa-ros  self  -moving; 
applied  to  movement  in  which 
the  body  acts  like  a  machine, 
without  apparent  intervention 
of  consciousness. 

Axis,  a  pivot ;  the  second  cervical 
vertebra. 

Bacillary,  latillum  a  little  staff; 
bacillary  layer  of  the  retina, 
consisting  of  rods  and  cones. 

Bicuspid,  bis  twice,  and  cuspis  a 
pointed  extremity;  two  pointed, 
as  the  bicuspid  teeth,  and  the 
bicuspid  valve  guarding  the  left 
auriculo-ventricular  opening  of 
the  heart. 

Biology,  j3i'o§  life,  and  Xoyos  dis- 
course ;  the  science  treating  of 
living  bodies. 

Blastoderm,  ft\aan-6<s  a  shoot  or 
germ,  and  <5e'f>/xa  a  skin ;  the 
germinal  membrane  in  which 
the  embryo  appears. 

Branchial,  branchiae  gills ;  belong- 
ing to  gills. 

Bronchus,  (3p6y^o^  the  wind-pipe ; 
technically  a  name  given  only 
to  each  of  the  two  tubes  into 
which  the  trachea  divides. 

Bronchia,  or  bronchial  tubes,  the 
smaller  tubes  into  which  the 
bronchi  divide. 

Buccal,  bucca  the  hollow  interior 
of  the  cheeks  ;  belonging  to  the 
cavity  of  the  mouth. 

Bursa,  a  pouch ;  a  membranous 
sack  interposed  between  parts 
which  are  subject  to  movement 
one  on  the  other,  to  allow  them 
to  glide  smoothly. 

Cadaveric,  cadaver  a  dead  body  ; 
cadaveric  rigidity  is  the  stiff- 
ness after  death. 

Csecum,  ccecu&blind. ;  the  blind  in- 
testine, intestinum  ccecum  or 
caput  ccecum  coll. 

Calcaneum,  calx  the  heel;  the 
heel  bone. 

Camera,  a  chamber. 

Canthus,  the  corner  of  the  eye. 

Capillary,  capillus  hair ;  capillary 


GLOSSARY. 


305 


vessels,  those  of  the  minutest 
order ;  capillary  blood-vessels, 
those  between  the  arteries  and 
veins ;  capillary  lymphatics, 
those  which  make  the  network 
of  origin  in  the  tissues. 

Caput  caecum  coli,  the  blind  head 
of  the  colon. 

Carbonaceous,  possessing  carbon ; 
applied  to  organic  substances 
which  contain  no  nitrogen. 

Carotid,  Kapa  the  head,  and  ous 
the  ear;  the  name  of  arterial 
trunks  ascending  to  the  head, 
close  to  the  ears. 

Carpus,  KapTTo's  the  wrist ;  the 
eight  small  bones  at  the  wrist. 

Cartilage,  cartilage  gristle. 

Caruncula,  caro  flesh;  a  little 
fleshy  mass. 

Cauda  equina,  horse  tail ;  the  col- 
lection of  large  nerves  descend- 
ing from  the  lower  end  of  the 
spinal  cord. 

Caudal,  belonging  to  the  tail,  as 
caudal  vertebrae. 

Cerebellum  little  brain  ;  the  part 
of  the  brain  overhanging  the 
medulla  oblqngata. 

Cerebrum  brain  ;  technically  ap- 
plied to  the  part  of  the  brain 
above  the  cerebellum  and  pons 
Varolii. 

Cervical,  cervix  the  neck  ;  belong- 
ing to  the  neck,  as  cervical  ver- 
tebrae. 

Cerumen,  cera  wax ;  the  wax  of 
the  ears. 

Chalaza,  x"^a£«  hail,  a  pimple ; 
the  string  which  suspends  the 
yolk  of  an  egg. 

Chlorophyll,  x^wP°'ff  green,  and 
<pv\\ov  a  leaf ;  the  green  colour- 
ing matter  found  in  leaves. 

Cholesterin,  x°^  bile,  and  ar^-eap 
suet ;  a  greasy;  substance  which 
crystallizes  in  quadrilateral 
scales,  found  in  bile,  and  ob- 
tained by  decomposition  of 
various  textures,  especially  brain 
substance. 

Chondrin,  x°V<5pos  cartilage ;  a  sub- 
stance nearly  allied  to  gelatin, 
obtained  from  cartilage. 

Chorda  dorsalis,  dorsal  cord ;   an 

H 


embryonic  structure  in  the  posi- 
tion afterwards  occupied  by  the 
bodies  of  the  vertebrae. 

Chorda  tympani,  cord  of  the  tym- 
panum ;  a  nerve  which  traverses 
the  tympanum  of  the  ear. 

Chordse  tendine3e,tendinous  cords; 
the  threads  which  retain  in  posi- 
tion the  cusps  of  the  auriculo- 
ventricular  valves  of  the  heart. 

Chorion,  x°Ptou  skin  ;  the  outer 
membrane  surrounding  the 
foetns. 

Choroid,  x°P°'s  &  choir,  and  tl&o? 
form  ;  a  structure  formed  of 
numbers  of  small  blood  vessels 
combined,  as  the  choroid  plex- 
uses of  the  brain,  and  choroid 
tunic  of  the  eye. 

Chyle,  x^Xo's  juice  ;  the  substance 
taken  up  by  the  lacteals. 

Chyme,  x^'w  I  pour  ;  the  pulp  sent 
from  the  stomach  into  the  in- 
testine. 

Cicatricula,  a  little  cicatrix,  the 
part  of  the  yolk  of  a  bird's  egg 
in  which  the  embryo  appears. 

Cicatrix,  a  scar. 

Cilium,  an  eyelash ;  a  lash  such  as 
those  which  keep  constantly 
moving  on  some  nucleated  cor- 
puscles. 

Circumvallate,  circum  around, 
and  vallum  a  rampart ;  sur- 
rounded by  a  rampart,  as  certain 
papillae  of  the  tongue  are. 

Clavicle,  clavis  a  key ;  the  collar 
bone. 

Coccyx,  a  cuckoo ;  the  caudal 
vertebrae  of  man,  named  from 
being  united  to  form  a  structure 
like  a  cuckoo's  beak. 

Cochlea,  a  snail's  shell ;  the  spiral 
part  of  the  labyrinth  of  the  ear. 

Colloid,  Ko\\a  glue,  and  eloo? 
form;  like  glue  or  gum.  In 
chemistry,  a  substance  whose 
solutions  are  imperfect  and  in- 
diffusible  through  membranes. 

Colon,  KU)\ov  a  member  of  the 
body  ;  the  great  intestine. 

Colostrum,  the  first  milk  after  the 
birth  of  the  child. 

Coma,  Kwfjia.  a  deep  sleep  ;  uncon- 
sciousness from  morbid  causes 


306 


GLOSSARY. 


other  than  deficient  pressure  of 
blood  on  the  brain. 

Commissure,  commissura  a  join- 
ing ;  a  connecting  link  between 
two  parts  in  the  cerebro-spinal 
axis. 

Concha,  a  shell ;  the  cup  of  the 
ear. 

Conjunctiva  (membrana),  COD  join- 
ing membrane ;  the  mucous 
membrane  folded  over  the  front 
of  the  eye-ball  and  the  interior 
of  the  eye-lids,  joining  them  to- 
gether. 

Condyle,  KovSuXos  a  knuckle ;  usu- 
ally applied  to  a  prominent  ar- 
ticular surface. 

Coracoid,  K<>pa£  a  crow,  and  el<5os 
form ;  shaped  like  a  crow's 
beak,  coracoid  process  of  scap- 
ula. 

Corium,  skin  ;  a  name  for  the 
cutis  vera. 

Cornea  (tunica),  the  corneous  or 
horny  tunic  ;  the  transparent 
part  of  the  outer  tunic  of  the 
eye-ball. 

Cornu,  a  horn  ;  applied  to  things 
projecting  like  horns,  as  the 
cornua  of  the  ventricles  of  the 
brain,  and  of  the  grey  matter 
of  the  spinal  cord. 

Coronoid,  /co^u>i/j]  a  crow,  and  e7<5os 
form  ;  like  a  crow's  beak  ;  thus 
coronoid  process  of  ulna  and  of 
lower  jaw. 

Corpora  albicantia,  whitish 
bodies  ;  a  pair  of  bodies  on  the 
base  of  the  brain. 

Corpora  CLuadrigemina,  bodies 
four  at  a  birth  ;  a  part  of  the 
brain  exhibiting  four  elevations. 

Corpora  striata,  striped  bodies ;  a 
pair  of  structures  in  the  brain. 

Corpus  callosum,  hard  body  ;  the 
great  transverse  commissure  of 
the  cerebral  hemispheres. 

Corpus  luteum,  yellow  body;  a 
body  found  in  the  ruptured 
Graafian  vesicle,  from  which  the 
ovum  has  escaped. 

Cortical,  cortex  bark ;  applied  to 
outside  portions  of  organs. 

Costal,  costa  a  rib  ;  belonging  to  a 
rib. 


Cribriform,  cribrum  a  sieve ;  per- 
forated with  small  holes. 

Cricoid,  /cpk-o?  a  ring,  and  £l<5os 
form ;  the  name  of  the  lower 
cartilage  of  the  larynx. 

Cruorin,  cruor  gore ;  the  colouring 
matter  of  the  blood. 

Crus,  a  leg,  shank,  or  column  of 
support ;  thus,  the  crura  cerebri, 
crura  cerebelli,  and  crura  of  the 
fornix. 

Crusta  petrosa,  stony  crust ;  a 
substance  allied  to  bone,  coat- 
ing the  fangs  of  teeth,  and  in 
many  animals  filling  depressions 
in  the  enamel. 

Crystalloid,  Kpva-TaX.Xo's  crystal, 
and  tldos  form ;  a  substance, 
the  solutions  of  which  are  dif- 
fusible through  membranes, 
such  substances  being  generally 
capable  of  crystallization. 

Cuboid,  /cu/3o5  a  cube,  and  «t<5o? 
form ;  the  name  of  a  bone  of  the 
foot. 

Cuneiform,  cuneus  a  wedge,  and 
forma  form ;  the  name  of  one 
bone  in  the  hand  and  of  three 
bones  in  the  foot. 

Cusp,  cuspis  a  spear-point,  or  other 
pointed  extremity ;  thus,  the 
cusps  of  the  crowns  of  teeth, 
and  of  the  auriculo- ventricular 
valves. 

Cutis  vera,  true  skin;  the  in- 
tegument beneath  the  cuticle. 

Cyanosis,  KUUVOS  dark  blue ;  a 
blue-skinned  condition,  the  re- 
sult of  deficient  aeration  of  the 
blood,  in  consequence  of  patency, 
after  birth,  of  the  foramen  ovale 
between  the  auricles  of  the 
heart. 

Cystic,  /cue-Tie  a  bladder  ;  belong- 
ing to  a  bladder*,  as  the  cystic 
duct,  the  duct  of  the  gall 
bladder;  ajso,  haying  bladders 
or  cysts,  as  a  cystic  tumour. 

Decidua,  things  subject  to  falling 
or  to  be  shed;  growths  of  the 
mucous  membrane  of  the 
uterus  during  pregnancy ;  dis- 
tinguished as  qecidua  vera,  re- 
flexa,  and  serotina— the  true, 


GLOSSARY. 


307 


the  reflected,  and  the  late  de- 
cidua.  The  milk-teeth  are  called 
deciduous,  on  account  of  being 
shed. 

Decussation,  decussatio  a  cutting 
across  in  the  form  of  the  letter 
X ;  a  crossing  of  fibres  or  other 
structures,  at  right  angles,  and 
also  otherwise. 

Deglutition,  de  down,  and  glutlo 
I  swallow;  the  act  of  swallow- 
ing. 

Dentine,  dens  a  tooth ;  the  texture 
of  which  the  teeth  in  greater 
part  consist. 

Depuration,  de  away  from,  and 
purus  pure  ;  the  clearing  away 
of  impurities. 

Derma,  dep^a  a  skin ;  the  cutis 
vera. 

Desquamation,  de  and.  squama  a 
scale;  falling  away  of  scales. 

Diaphragm,  <5ia  across,  and  $>pay- 
M.O.  a  fence;  any  partition  (as  the 
diaphragm  of  a  microscope),  by 
means  of  which  the  aperture  for 
the  admission  of  light  is  dimin- 
ished ;  the  midriff  or  muscular 
partition  separating  the  thoracic 
from  the  abdominal  cavity. 

Diaphysis,  cid  right  through,  and 
</>uc7i9  growth  ;  the  centre  of  os- 
sification of  the  main  length  or 
shaft  of  a  long  bone. 

Dicrotism,  <5/'-:  twice,  and  Kporew 
I.  be&t ;  the  double  beating  of 
the  arterial  pulse. 

Digestion,  digero  I  divide;  the 
conversion  of  the  food  into  a 
substance  capable  of  absorption. 

Discus  proligerus,  disc  bearing  the 
offspring  ;  a  coating  of  granules 
on  the  ovum. 

Dorsal,  dor  sum  the  back ;  is  pro- 
perly used  in  opposition  to 
ventral ;  but  in  the  case  of  the 
dorsal  vertebrae,  it  is  applied  to 
the  twelve  vertebrae  which  bear 
the  ribs,  and  ought  to  be  sup- 
planted by  the  word  thoracic. 

Ductus    communis    choledochus, 
v-j\i7    bile,   and    oe'xo/icu   I    re- 
ceive ;  the  common  bile  duct. 
Duodenum,  duodeni  twelve;  the 
part  of   the  intestine  immedi- 


ately succeeding  the  stomach  ; 
named  from  being  considered 
about  twelve  finger -breadths 
long. 

Dura  mater,  hard  mother;  the 
name  of  the  tough  fibrous  cover- 
ing of  the  brain  and  spinal  cord. 

Electrotonus,  *j\6KTpov  amber, 
and  TOVO<S  tension ;  the  electric 
condition  into  which  a  nerve  is 
thrown  when  a  continuous  cur- 
rent of  electricity  passes  along 
any  part  of  its  course.  m 

Embryo,  fyppvov  (ev  within,  and 
/fyuco  I  swell) ;  the  young  before 
birth  ;  applied  principally  to 
the  very  early  stages  of  exist- 
ence, and  used  in  reference  to 
plants  as  well  as  animals. 

Embryology,  e/nftpvov,  and  Xo'yo? 
discourse ;  the  study  of  develop- 
ment. 

Emunctory,  emungo  I  wipe ;  any 
j>art  by  which  waste  matter  is 
got  rid  of. 

Enamel,  in  French  email,  in 
Italian  smalto,  /ue'\oo>  I  melt ;  a 
fused  substance  spread  on  a  sur- 
face ;  the  name  given  to  the  ex- 
ceedingly hard  texture  which 
covers  the  crowns  of  the  teeth. 

Encephalon,  lv  within,  and  /CE- 
<pa\v  the  head;  the  whole  brain 
down  to  where  the  medulla  ob- 
longata  is  continued  into  the 
spinal  cord. 

Endogenous,  ev&ov  within,  and 
yevvait)  I  bring  forth  ;  growing 
in  the  interior  of  the  pre-exist- 
ing structure. 

Endolymph,  ev&ov  within,  and 
lympha  water  ;  the  fluid  within 
the  membranous  labyrinth. 

Endosmosis,  eV<W  within,  and 
wOe'w  I  push  ;  the  current  from 
without  inwards,  when  diffusion 
of  fluids  takes  place  through  a 
membrane. 

Endothelium,  evdov  within,  and 
QdXXa)  I  bloom  ;  an  exceedingly 
delicate  coating  of  squamous 
corpuscles  found  in  the  interior 
of  capillary  blood-vessels  and 
lymphatics, 


308 


GLOSSARY. 


Epidermis,  e-n-i  upon,  and  Sep/xa 
the  skin  ;  the  cuticle  or  scarf 
skin. 

Epigastrium,  CTTL  upon,  and  yaa- 
Tt'ip  the  belly ;  the  upper  abdo- 
minal region,  below  the  sternum 
and  between  the  costal  carti- 
lages of  opposite  sides. 

Epiglottis,  £7ri  upon,  and  f/lotti*; 
the  cartilaginous  lid  which  lies 
above  the  glottis. 

Epiphysis,  kiri  upon,  and  (j>v<ns 
growth ;  a  supplementary  centre 
of  ossification,  such  as  those 
found  at  the  extremities  of  long 
bones  and  at  the  tips  of  the 
spinous  and  transverse  processes 
of  vertebrae. 

Epithelium,  eiri  upon,  and  0^A.Xo> 
I  bloom ;  a  coating  of  one  or 
more  strata  of  nucleated  cor- 
puscles on  a  free  surface. 

Ethmoid,  ?/6/xo5  a  sieve,  and  eldos 
form ;  one  of  the  bones  of  the 
head,  so  named  from  being  per- 
forated with  a  number  of  little 
holes,  through  which  the  fila- 
ments of  the  nerve  of  smell 
pass. 

Excretin,  excreta  things  excreted ; 
a  crystalline  substance  obtained 
from  faeces. 

Excretion,  ex  out,  and  cresco  1 
grow;  any  waste  material  thrown 
out  from  the  body. 

Exogenous,  e£w  without,  and  yev- 
vaw  I  bring  forth  ;  growing  out- 
side the  pre-existing  structure. 

Exosmosis,  e£w  without,  and  &0ew 
I  push  ;  the  current  from  with- 
in outwards,  when  diffusion  of 
fluids  takes  place  through  a 
membrane. 

Faeces,  dregs ;  the  discharges  by 

the  bowel. 

Falx,  a  sickle  ;  falx  cerebri  a  pro- 
cess of  dura  mater  between  the 

cerebral  hemispheres. 
Fascia,  a  bandage ;   felted  white 

fibrous  tissue  disposed  in  the 

form  of  a  membrane. 
Fasciculus,    a    little    bundle,    as 

those   of   muscular  and  nerve 

fibres. 


Fauces,  faux  the  gullet ;  the  pas- 
sage beneath  the  soft  palate,  be- 
tween the  mouth  and  pharynx. 

Femur,  the  thigh ;  the  thigh  bone. 

Fenestra  ovalis  and  fenestra  ro- 
tunda, the  oval  and  the  round 
window;  two  apertures  in  the 
bone  between  the  tympanic 
cavity  and  the  labyrinth  of  the 
ear. 

Fibrilla,  a  little  fibre;  one  of  the 
longitudinal  threads  into  which 
a  striped  muscular  fibre  can  bo 
divided. 

Fibrin,  fibra  a  fibre  ;  a  variety  of 
albuminoid  substance,  named 
from  its  coagulating  and  ex- 
hibiting a  fibrous  structure. 

Fibrinogen,  fibrin,,  and  ytwaia  I 
bring  forth;  the  substance  in 
the  blood  and  elsewhere  which 
coagulates  on  addition  of  fibrino- 
plastin. 

Fibrinoplastin,yz6rm,  and  TrXao-o-co 
I  fashion  ;  the  substance  which, 
added  to  fibrinogen,  causes  it 
to  coagulate. 

Fibula,  a  clasp  or  buckle  ;  the 
small  or  outer  bone  of  the 
leg. 

Filiform,  filum  a  thread,  and 
forma  form  ;  thread-like,  as  the 
filiform  papillae  of  the  tongue. 

Filum  terminate,  the  terminal 
thread  which  descends  from  the 
extremity  of  the  spinal  cord. 

Fimbriated,  fimbria  a  fringe  • 
fringed. 

Fissiparous,  fissus  cleft,  and  pario 
I  bring  forth ;  multiplying  by 
division  into  equal  parts 

Foetus,  the  young  of  any  animal ; 
the  unborn  offspring. 

Follicle,  folliculus  diminutive  of 
follis  a  bag ;  a  simple  gland  or 
other  pouch,  as  the  follicles  of 
LieberkuhrLand  closed  follicles. 

Foramen,  a  hole. 

Fornix,  an  arch;  a  part  of  the 
brain. 

Fossa,  a  ditch  ;  a  depression,  par- 
ticularly in  a  bone. 

Fovea  centrally  the  central  pit ; 
a  part  of  the  retina. 

Frontal,  Jrons  the  forehead  ;  the 


GLOSSARY. 


309 


name  of  the  bone  which  forms 
the  forehead. 

Fundus,  the  bottom  of  anything ; 
used  with  reference  to  hollow 
viscera,  such  as  the  uterus  and 
bladder. 

Fungiform,  fungus  a  mushroom; 
thicker  at  the  extremity  than 
at  tie  attached  part,  as  the 
fungiform  papillae  of  the  tongue. 

Ganglion,  both  Greek  and  Latin, 
a  swelling  or  hard  knot  or 
lump;  in  anatomy,  a  swelling 
on  a  nerve,  and  any  nervous 
centre. 

Ganglion  impar,  ganglion  without 
fellow ;  the  mesially  situated 
lowest  ganglion  of  the  sym- 
pathetic chain. 

Galvanometer,  galvanism  (from 
Gdlyani),  and  /ueh-poi/  a  measure; 
an  instrument  which  indicates 
the  presence,  direction,  and 
strength  of  a  galvanic  current 
by  the  deviations  of  a  magnetic 
needle. 

Gastrocnemius,  yac-rvp  a  belly, 
and  Kvi'ifjit]  the  leg ;  a  muscle 
named  from  forming,  in  part, 
the  swelling  of  the  calf  of  the  leg. 

Gelatin,  gelo  I  freeze ;  a  nitro- 
genous substance,  obtained  by 
boiling  integument  and  other- 
tissues,  the  solutions  of  which 
form  a  jelly  on  cooling. 

Gemmiparous,  gemma  a  bud.  and 
pario  I  bring  forth  ;  repro- 
ducing by  buds.  ui 

Ginglymus,  yt-yyXu^os  a  hinge  ;  a 
joint  which  admits  of  move- 
ment in  only  one  plane — that  is, 
flexion  and  extension. 

Glenoid,  yXrivi]  the  pupil,  or  a 
shallow  depression,  and  el£os 
form.  The  glenoid  fossa  is  the 
name  given  to  the  articular  sur- 
face of  the  scapula. 

Glomerulus,  glomus  a  clew  of 
thread ;  the  clump  of  vessels 
within  a  Malpighian  corpuscle 
of  the  kidney. 

Glottis,  yXwTTa  the  tongue  ;  the 
aperture  into  the  wind-pipe,  be- 
tween the  vocal  cords. 


more      properly 
blood, 


Glycpgen,  yXu/cus  sweet,  ytwaw  I 
bring  forth  ;  a  substance  formed 
in  the  liver,  convertible  into 
grape  sugar  or  glucose  ;  called 
also  amyloid  substance. 

Gyri  operti,  hidden  convolutions  ; 
another  name  for  the  Island  of 
Reil  ;  the  convolutions  at  the 
bifurcation  of  the  fissure  of 
Sylvius. 

Haemoglobin, 

hcematoglobulin,  a/xa  , 

and  globulin  (globulus  a  globule)  ; 
the  globulin  of  the  blood  ;  a 
variety  of  albuminoid  substance, 
characteristic  of  the  red  blood 
corpuscles. 

Haematin,  at/ma  blood  ;  an  in- 
soluble substance,  containing  in 
an  altered  form  the  colouring 
matter  of  the  blood. 

Helicotrema,  e/\i£  a  spiral,  and 
Tprjua  a  hole;  the  opening  by 
which  the  two  scalse  communi- 
cate at  the  summit  of  the 
cochlea. 

Helix,  e\i%  a  spiral  ;  the  elevation 
which  forms  the  greater  part  of 
the  margin  of  the  outer  ear. 

Hepatic,  Tj-rrap  the  liver  ;  belong- 
ing to  the  liver. 

Hippocampus,  I'TTTTOS  a  horse,  and 
KctfjiTni  a  bending  ;  a  fish  with  a 
head  like  a  horse,  and  a  curly 
tail;  the  name  of  certain  curved 
structures  in  the  brain. 

Hilus,  hilum  the  mark  on  the  con- 
cavity of  a  bean  ;  the  concave 
part  of  the  kidney  where  the 
ureter  emerges. 

Histology,  IO-TOS  a  web,  and  Xo'yos 
discourse  ;  the  study  of  the  tex- 
tnres. 

Homologous,  O/ULOIOS  similar,  and 
Xdyos  a  word  ;  similar  in  struc- 
ture, or  having  structural  af- 
finity, as  contradistinguished 
from  similarity  of  function. 

Hyaline  and  Hyaloid,  OaXos  crys- 
tal ;  clear  as  crystal. 

Hyoid,  «',  and  el<5os  form  ;  U 
shaped  ;  the  name  of  the  bone 
above  the  larynx. 

Hypoaria,  UTTO  under  ;   a  pair  of 


310 


GLOSSARY. 


bodies  on  the  under  surface  of 
the  brain  in  tishes. 

Hypochondrium,  u-n-6  under,  and 
XoV<5pos  cartilage ;  the  upper 
lateral  region  of  the  abdomen, 
under  cover  of  the  costal  carti- 
lages. 

Hypogastrium,  VTTO  under,  and 
yaarvp  the  belly ;  the  lower 
mesial  region  of  the  abdomen. 

Hypoglossal,  VTTO  under,  and 
yXuHTcra  the  tongue ;  under  the 
tongue;  the  name  of  the  last 
cranial  nerve. 

Ileum,  et\co  or  f\\w  I  twist ;  the 
lower  three-fifths  of  the  small 
intestine. 

Ilium,  d\eta  T  twist :  the  upper 
division  of  the  os  innominatum. 

Imbricated,  imbrex  a  roof-tile ; 
sloped  one  over  another,  like 
tiles. 

Incisor,  incido  lent;  incisor  (dens) 
a  cutting  tooth. 

Incus,  in  and  cudo  I  hammer  ;  an 
anvil ;  one  of  the  ossicles  in  the 
tympanum. 

Infundibulum,  a  funnel ;  a  hollow 
process  descending  from  the 
third  ventricle  of  the  brain  to 
the  pituitary  body. 

Innominate,  nomen  a  name ;  un- 
named ;  innominate  artery  and 
innominate  bone. 

Intercostal,  inter  between,  and 
costa  a  rib ;  between  two  suc- 
cessive ribs. 

Invagination,  vagina  a  sheath ; 
the  pushing  of  one  part  of  a  hol- 
low structure  into  the  interior 
of  another  part,  as  may  be 
done  with  the  finger  of  a  glove. 

Involution,  volvo  I  roll ;  rolling 
in ;  backward  growth,  such  as 
the  return  of  the  uterus  after 
parturition  to  its  ordinary  di- 
mensions. 

Iris,  a  rainbow ;  the  coloured  cur- 
tain in  the  eye. 

Ischium,  Icrxiov  the  hip ;  the  lower 
and  hinder  division  of  the  in- 
nominate bone,  on  which  we  sit. 

Jejunum,  empty;  the  upper  two- 


fifths  of  the  small  intestine  suc- 
ceeding the  duodenum. 

Jugal,  jugum  a  yoke ;  another 
name  for  the  malar  or  cheek 
bone. 

Jugular,  jugulum  the  fore  part  of 
the  neck  ;  the  name  given  to 
certain  large  veins  in  the  neck. 

Kreatin,  Kpias  flesh ;  a  soluble 
nitrogenous  substance  contained 
in  flesh,  and  jirobably  a  pro. 
duct  of  decomposition  of  al- 
buminoid substance. 

Lachrymal,  labliryma  a  tear ;  hav- 
ing to  do  with  the  tears  ;  as 
lachrymal  gland. 

Lacteals,  lac  milk  ;  the  absorbent 
vessels  of  the  small  intestine, 
named  from  the  milky  appear- 
ance of  the  chyle  which  they 
convey. 

Lacuna,  a  wet  ditch  or  hollow  ;  a 
microscopic  hollow  in  the  mat- 
rix of  bone,  occupied  in  the 
recent  state  by  a  bone  cor- 
puscle. 

Laryngoscope,  \apvy£  and  tncov^w 
I  behold  ;  an  instrument  con- 
sisting of  a  mirror  held  in  the 
throat,  and  a  reflector  to  throw 
light  on  it,  by  which  the  in- 
terior of  the  larynx  is  brought 
into  view. 

Larynx,  \dpwy%  the  upper  part  of 
the  wind-pipe,  extending  down 
to  the  lower  border  of  the  cri- 
coid  cartilage. 

Lenticular,  lens  or  lenticula  a  len- 
til; the  closed  follicles  of  the 
stomach  are  called  lenticular. 

Leucocyte,  XtvKos  white,  /CUT-OS  a 
hollow ;  a  white  blood  cor- 
puscle ;  an  objectionable  word, 
seeing  that  those  corpuscles  are 
not  hollow  cells. 

Leucocythemia,  \eu/co?,  K-UT09,  and 
alfjia  blood  ;  a  malady  in  which 
the  number  of  white  corpuscles 
in  the  blood  is  greatly  increased. 

Ligament,  ligamentum  a  band,  llgo 
I  bind  ;  a  band  uniting  two 
structures,  usually  bones. 

Liquor    sanguinis,    fluid   of  the 


GLOSSARY. 


311 


blood;  the  blood  minus  the 
corpuscles. 

Locule,  locidus  (diminutive  of 
locus)  a  little  space;  a  minute 
hollow. 

Lumbar,  lumbus  the  loin  ;  belong- 
ing to  the  loins,  as  lumbar  re- 
gion, lumbar  vertebrae. 

Lymph,  lymplia  water  ;  the  col- 
ourless fluid  brought  back  from 
the  textures  by  special  absorb- 
ent vessels  called  lymphatics; 
also  used  in  pathology  to  de- 
note clear  coagulable  substance 
thrown  out  from  the  textures 
in  abnormal  circumstances. 

Malar,  mala  the  prominence  of 
the  cheek ;  the  name  of  the 
cheek-bone. 

Malleus,  a  hammer;  one  of  the 
ossicles  in  the  tympanum  of  the 
ear. 

Mamma,  the  breast. 

Mandible,  mandibula  (mando  I 
chew)  the  lower  jaw. 

Manubrium,  a  handle  ;  manubri- 
urn  of  the  malleus  and  of  the 
sternum. 

Mastication,  /xacrao/xat  I  chew ; 
the  whole  mechanical  breaking 
up  of  the  food  in  the  mouth. 

Mastoid,  /X«O-TOS  a  breast,  and 
el£o5  form;  nipple  shaped;  the 
name  of  the  process  of  the  tem- 
poral bone,  behind  the  ear. 

Matrix,  a  womb ;  the  substance 
in  which  anything  is  embedded. 

Maxilla,  a  jaw  ;  applied  to  both 
jaws,  the  bones  being  respec- 
tively called  superior  and  in- 
ferior maxillary. 

Meatus,  a  passage ;  external  and 
internal  auditory  meatus  ;  and 
superior,  middle,  and  inferior 
meatus  of  the  nose. 

Meconium,  HI'IKMV  a  poppy;  poppy 
juice;  the  faeces  of  the  new- 
born infant. 

Medulla  oblongata,  elongated 
marrow  ;  the  part  of  the  brain 
continuous  with  the  spinal  cord. 

Medullary,  medulla  marrow ;  usu- 
ally applied  to  central  parts  of 
organs,  in  opposition  to  corti- 
cal ;  but  the  medullary  sheath 


of  a  nerve  fibre  is  named  from 
having  a  consistence  like  mar- 
row. 

Meninges,  /uT/i/tyg  a  membrane;  a 
name  given  to  the  membranes 
of  the  brain  and  spinal  cord. 

Mesentery,  /meo-oe.  middle,  and  eV 
Ttpoi/  intestine;  structure  form- 
ed by  duplication  of  peritoneum, 
intervening  between  intestine 
and  abdominal  wall. 

Metacarpus,  //era  after,  and  r«p- 
0-05  the  wrist ;  the  five  bones  be- 
yond the  carpus,  and  support- 
ing the  digits  of  the  hand. 

Metatarsus,  /ULETO.  after,  and  T«f>- 
TTO'S  the  flat  of  the  foot ;  the 
five  bones  beyond  the  tarsus, 
and  supporting  the  toes. 

Mitral,  like  a  mitre ;  a  name  given 
to  the  bicuspid  valve  of  the 
heart. 

Modiolus,  a  nave  of  a  wheel;  the 
central  column  round  which  the 
cochlea  winds. 

Molar,  mola  a  mill;  a  grinder  or 
back  tooth. 

Morphology,  fJLoptfnj  form,  and 
XJyos  discourse ;  the  study  of 
the  laws  of  form  or  structure  in 
living  beings. 

Mucus,  discharge  from  the  nose ; 
any  such  viscid  secretion;  con- 
taining a  peculiar  nitrogenous 
substance,  mucin. 

Multicuspid,  multus  many,  and 
cuspis  a  pointed  extremity.  The 
molar  teeth  are  multicuspid, 
having  several  cusps  on  their 
crowns. 

Musculi  papillares,  papillary 
muscles  ;  the  muscular  projec- 
tions in  the  interior  of  the  ven- 
tricles of  the  heart,  to  which 
the  chordae  tendiuese  of  the  auri- 
culo- ventricular  valves  are  at- 
tached. 

Myographion,  /nv-s  a  muscle,  and 
ypa(/>o>  I  write ;  an  instrument 
by  which  the  rapidity  of  the 
nervous  current  is  determined 
by  the  time  at  which  a  muscle 
contracts  after  application  of 
stimuli  to  different  parts  of  the 
course  of  the  nerve  supplying  it. 


312 


GLOSSARY. 


Myosin,  /uu?  a  muscle;  muscle- 
fibrin  obtained  from  living 
muscle. 

Nares,  the  nostrils ;  anterior  nares 
the  nostrils  proper ;  posterior 
wares  the  openings  of  the  nasal 
cavities  into  the  pharynx. 

Necrosis,  ye/cpo's  a  dead  body ;  the 
death  of  a  mass  of  bone. 

Nitrogenous,  containing  nitrogen ; 
organic  substances  containing 
nitrogen  are  so  called. 

Notochord,  I/OITOS  the  back,  and 
XopM  a  string;  the  embryonic 
structure  round  which  the 
bodies  of  the  vertebrae  are  de- 
veloped, called  also  chorda  dor- 
sails. 

Nucha,  an  un classical  word  for 
the  neck;  ligamentum  nucJice, 
the  ligament  which  in  many 
animals  suspends  the  head  from 
the  spines  of  the  vertebrae. 

Nucleus,  a  kernel;  a  firm  albu- 
minoid structure  in  the  interior 
of  a  living  corpuscle,  or  such  a 
structure  in  a  matrix,  though 
no  corpuscle  be  seen  to  surround 
it. 

Nucleolus,  diminutive  of  nucleus; 
a  dense  body  within  the  sub- 
stance of  a  nucleus. 

Occipital,  occiput  (ob  and  caput) 
the  hinder  part  of  the  head ; 
the  name  of  the  hindermost 
bone  of  the  skull. 

Ocellus,  diminutive  of  oculus,  a 
little  eye  ;  applied  to  a  minute 
eye  or  a  unit  of  a  compound  eye 
in  the  invertebrata. 

Odontoid,  o<5ous  a  tooth,  and  £t<5os 
form  ;  like  a  tooth  ;  the  name 
of  the  process  surmounting  the 
body  of  the  second  cervical  ver- 
tebra. 

(Esophagus,  o'tcru)  future  of  <pepw 
I  bear,  and  tyayelv  to  eat ;  the 
bearer  of  things  eaten  ;  the  di- 
gestive tube  from  the  point  at 
which  it  becomes  a  completely 
separate  tube  at  the  termina- 
tion of  the  pharynx,  down  to 
the  entrance  of  the  stomach. 


Olecranon,  wXe'i/i;  the  elbow,  and 
Kpaviov  the  top  of  the  head ;  the 
summit  of  the  ulna. 

Olivary,  like  an  olive ;  olivary 
eminence  of  medulla  oblongata, 
and  olivary  process  of  the 
sphenoid  bone. 

Omasum  (w/uo's  raw)  a  tripe ;  the 
third  stomach  of  a  ruminant, 
called  in  French  feuillet. 

Omphalo-mesenteric,  d/uupaXo's  the 
ravel  (see  Mesentery) ;  the  name 
of  vessels  in  the  young  foetus 
which  return  blood  from  the 
walls  of  the  umbilical  vesicle. 

Operculum,  a  lid.  The  qpercula 
of  the  tooth  sacs  cover  in  those 
cavities. 

Ora  serrata  (serra  a  saw),  the  ser- 
rated margin ;  the  anterior 
border  of  the  retina,  so  called 
from  its  serrated  appearance  in 
the  human  eye. 

Orbit,  orbit  a  the  tract  in  which 
anything  rolls ;  the  socket  of 
the  eyeball. 

Organ,  opyavov  an  instrument 
(epyov  work). 

Organic  world,  all  structures  hav- 
ing organs ;  namely,  animals 
and  vegetables. 

Organic  matter,  such  chemical 
compounds  as  are  derived  from 
the  organic  world. 

Organic  functions,  processes  of 
nutrition,  independent  of  con- 
sciousness ;  in  contradistinction 
to  animal  functions,  in  which 
consciousness  is  involved. 

Organism,  a  being  with  organs  ; 
any  plant  or  animal. 

Organized,  having  organs ;  diff- 
erentiated structure. 

Os  magnum,  large  bone  ;  the  larg- 
est of  the  eight  carpal  bones. 

Os  uteri,  mouth  of  the  womb. 
The  os  externum  and  internum 
are  distinguished  at  the  lower 
and  upper  end  of  the  cervix. 

Osmosis,  wdeo)  I  push ;  the  dif- 
fusion of  fluids  through  mem- 
branes. 

Ossification,  os  bone,  and  fado  I 
make  ;  the  formation  of  bone. 

Osteoblastic,    cxrreov    bone,    and 


GLOSSARY. 


313 


/3Xa<TTt'a>  I  shoot  up ;  osteoblastic 
corpuscles,  those  from  which 
bone  is  immediately  formed. 

Otoconia,  ous  an  ear,  and  Kovia 
dust ;  minute  hard  particles  in 
the  vestibule  of  the  ear. 

Otolith,  ous  an  ear,  and  Xi'Oos  a 
stone  ;  used  instead  of  otoconia; 
also  applied  to  larger  bodiesfound 
in  the  ears  of  some  animals,  as 
fishes,  to  which  the  name  oto- 
conia would  be  inapplicable. 

Ovum,  an  egg ;  applied  only  to  a 
germ  which  requires  impregna- 
tion before  being  developed. 

Palatal,  palatus;  the  name  of  a 
bone  which,  besides  completing 
the  wall  of  the  nasal  cavity, 
forms  the  back  part  of  the  hard 
palate,  behind  the  superior 
maxillary. 

Pancreas,  Tray  all,  and  Kpeas  flesh ; 
the  organ  called  by  butchers 
the  sweetbread. 

Papilla,  a  nipple.  The  papillae  of 
the  skin  are  the  minute  eleva- 
tions into  which  the  cutis  vera 
is  thrown. 

Paralysis,  Trapa-Xv^  I  loosen  from 
beside ;  the  loss  of  nervous 
power,  either  motor,  sensory,  or 
both. 

Parietal,  paries  a  wall.  The  pari- 
etal layers  of  serous  membranes 
are  those  lining  the  walls  of  the 
cavities  within  which  the  vis- 
cera which  they  surround  are 
situated  ;  parietal  bones,  those 
which  form  the  middle  part  of 
the  roof  of  the  skull. 

Parotid,  Trapd  beside,  and  ou§  the 
ear ;  the  name  of  the  salivary 
gland  which  lies  between  the 
lower  jaw  and  the  ear. 

Parthenogenesis,  -jrapflei/os  a  vir- 
gin, and  yevf.Gi<s  birth ;  repro- 
duction by  means  of  au  unim- 
pregnated  germ. 

Patella,  a  dish  ;  the  knee-pan. 

Pelvis,  a  basin  ;  the  cavity  bound- 
ed by  the  ossa  innominata  and 
sacrum.  The  part  below  the 
line  extending  round  from  the 
base  of  the  sacrum  to  the  sym- 


physis  pubis  is  the  true  pelvis; 
the  part  between  the  expanded 
blades  of  the  iliac  bones  is  the 
false  velvis. 

Pepsin,  TreWo)  or  TTCTTTW  to  cook  or 
digest ;  the  active  principle  of 
the  gastric  juice. 

Peptone,  TreVTw;  a  nitrogenous  sub- 
stance rendered  by  action  of  the 
gastric  juice  fit  for  absorption. 

Pericardium,  napi  around,  and 
KapSta  the  heart ;  the  serous  in- 
vestment of  the  heart,  with  the 
fibrous  bag  in  which  it  is  con- 
tained. 

Perichondrium,  irepi  around,  and 
XovSpos  cartilage ;  a  fibrous 
membrane  containing  blood- 
vessels, surrounding  cartilage. 

Perilymph,  Trtpi  around,  and  lym- 
pha  water;  the  fluid  in  which 
the  membranous  labyrinth  of 
the  ear  is  suspended. 

Periosteum,  ^repi,  and  oo-reoi/ 
bone ;  the  fibrous  membrane 
surrounding  a  bone,  and  con- 
taining the  ramifications  of 
arteries,  small  twigs  of  which 
penetrate  into  the  interior. 

Periphery,  -Trcpi,  and  (pep to  I  bear ; 
circumference ;  the  surrounding 
parts  as  contrasted  with  any 
centre.  Thus  the  spinal  nerves 
are  called  peripheral  nerves,  as 
contrasted  with  the  nerve  fibres 
within  the  brain  and  spinal  cord. 

Peristaltic,  irtpt9  and  o-Te'XAeo  I 
dispose  ;  the  name  given  to  that 
kind  of  movement  which  takes 
place  in  the  walls  of  the  intes- 
tine, the  wave  of  contraction 
embracing  the  viscus,  and 
travelling  onwards ;  likewise 
called  vermicular  movement. 

Peritoneum,  ir^i,  and  Teivw  I 
stretch  ;  the  serous  membrane 
of  the  abdominal  cavity. 

Petrous,  TreVpos  a  stone  ;  stony. 
The  basal  part  of  the  temporal 
bone  is  called  the  petrous  por- 
tion, on  account  of  its  hardness. 

Phalanx,  (pd\ay%  a  line  of  soldiers ; 
a  rank ;  a  bone  of  a  digit ; 
named  from  those  bones  being 
disposed  in  rows. 


314 


GLOSSARY. 


Pharynx,  </>apuy£  the  throat ;  the 
part  behind  the  nose,  mouth, 
and  larynx,  and  above  the  oeso- 
phagus. 

Phosphene,  </>£§  light,  and  (fraivo- 
/xai  I  appear ;  an  appearance  of 
light  produced  by  pressure  on 
the  eye-ball. 

Phrenic,  </>p??j/  the  midriff;  belong- 
ing to  the  diaphragm;  as  the 
phrenic  nerve. 

Phrenology,  Qpnv  the  mind,  and 
Xo'yos  discourse ;  a  study  of  the 
brain  or  skull,  with  a  view  to 
discovering  the  mental  qualities. 
The  particular  theory  originated 
by  Gall  is  usually  meant. 

Physiology,  <£uo-is  nature,  and 
Xo'yos  discourse ;  the  study  of 
the  operations  which  take  place 
in  living  beings. 

Pia  mater,  pious  mother ;  the 
vascular  covering  which  closely 
invests  the  brain  and  spinal  cord. 

Pineal,  pinea  a  pine.  The  pineal 
body  is  a  portion  of  the  brain 
in  front  of  the  corpora  quadri- 
gemina,  shaped  something  like 
a  fir  cone,  and  also  called  cona- 
rium, 

Pinna,  a  fin  or  a  pinion ;  the  ex- 
pansion of  the  external  ear. 

Pisiform,  pisum  a  pea  ;  the  name 
of  one  of  the  carpal  bones. 

Pituitary,  pituita  phlegm.  The 
pituitary  body  is  the  name  of  a 
structure  at  the  base  of  the 
brain,  formerly  looked  on  as  a 
gland. 

Placenta,  a  cake  ;  the  after-birth. 

Plasma,  TrXdo-^a  a  thing  modelled. 
Is  used  to  signify  material  from 
which  structure  is  formed ; 
thus,  blood  plasma  is  another 
name  for  liquor  sanguinis. 

Pleura,  TrXevpa  a  rib ;  the  serous 
membrane  which  invests  the 
lung. 

Plexus,  woven ;  a  set  of  nervous 
trunks  or  twigs  more  or  less 
closely  matted  together. 

Plica  sennilunaris,  semilunar 
fold ;  the  fold  of  mucous  mem- 
brane resting  on  the  inner  part 
of  the  eye-bail. 


Pneumo-gastric,  mte&jju&v  a  lung, 
and  yao-T-jJp  a  belly  ;  the  name 
of  a  cranial  nerve  which  sends 
branches  to  the  lung,  and  to  the 
stomach,  and  other  abdominal 
viscera. 

Pneumonia,  TTVCVIJLWV  a  lung ;  in- 
flamation  of  the  lungs. 

Portal  vein,  or  vena  portce,  vein 
of  the  gate  ;  the  gate  alluded  to 
being  the  transverse  fissure  of 
the  liver,  in  which  the  portal 
vein  divides  to  send  its  blood 
into  the  capillaries  of  the  liver. 
A  portal  system  is  an  expression 
applied  to  any  redistribution  of 
blood  from  a  vein  to  a  second 
set  of  capillaries. 

Premolar,  prc&  before  ;  in  front  of 
molars;  a  name  applied  to  the 
bicuspid  teeth. 

Process,  processus  a  going  for- 
ward ;  the  technical  term  for 
any  projection  of  bone  or  other 
tissue. 

Prostate,  pro  before,  and  status 
set;  the  name  of  a  gland  set  in 
front  of  the  orifice  of  the  urinary 
bladder  in  the  male. 

Protagon,  Trpwro?  first,  and  ayw 
I  lead  ;  the  name  of  a  chemical 
substance  obtained  from  brain 
matter  and  other  sources. 

Proteid,  TT/OCOTOS  first ;  derived  im- 
mediately from  protein,  a  sup- 
posed compound  radicle,  from 
which  Mulder  taught  that  the 
various  substances  of  the  al- 
buminoid group  were  all  de- 
rived. Proteid  is  therefore 
another  name  for  albuminoid; 
but  it  is  founded  on  an  erro- 
neous theory. 

Pseudoscope,  \l/ev8o*  a  lie  or 
cheat,  and  o-KoWw  I  behold  ;  an 
instrument  by  means  of  which 
hollow  objects  are  made  to  ap- 
pear projecting,  and  projecting 
objects  hollow. 

Pterygoid,  7rre'pu£  a  wing,  and 
sToos  form.  The  pterygoid  pro- 
cesses of  the  sphenoid  bone  pro- 
ject downwards  behind  the 
palate  bones,  with  which  they 
articulate. 


GLOSSARY. 


315 


Ptyalin,  TTTUW  I  spit ;  the  active 
principle  contained  in  saliva. 

Pulmonary,  pulmo  a  lung,  belong- 
ing to  the  lungs. 

Pus,  matter  from  a  sore. 

Pylorus,  7ru/\oujoos  ('TruXi'i  ovpos]  a 
gate-keeper ;  the  opening  from 
the  stomach  into  the  intestine, 
or,  more  properly,  the  struc- 
tures surrounding  the  opening. 

Eadius,  a  ray  or  spoke ;  the  outer 
bone  of  the  fore-arm. 

Receptaculum  chyli,  receptacle  of 
chyle ;  the  dilated  commence- 
ment of  the  thoracic  duct. 

Kectum,  for  intestinum  rectum 
straight  gut ;  the  last  part  of 
the  bowel.  In  many  mammal's 
it  passes  backwards  in  a  straight 
course  from  a  position  well  for- 
ward in  the  abdomen  ;  but  it  is 
by  no  means  straight  in  the 
human  subject. 

Renal,  ren  a  kidney,  belonging  to 
the  kidney. 

Eestiform,  restis  a  rope ;  the  name 
of  the  tracts  of  fibres  of  the 
medulla  oblongata,  which  pass 
into  the  cerebellum  ;  they  have 
slightly  spiral  marks,  like  a  rope, 
on  the  surface. 

Reticular,  rete  a  net;  in  meshes; 
as  is  the  matrix  of  reticular  car- 
tilage. 

Reticulum,  a  little  net;  the  name 
given  to  the  web  of  delicate  con- 
nective tissue  between  the  ner- 
vous elements  in  the  spinal 
cord  and  some  parts  of  the 
brain. 

Rete  mirabile,  wonderful  net;  a 
number  of  branches,  derived 
from  the  breaking  up  of  one  or 
more  arteries,  and  uniting  again 
into  larger  trunks. 

Rete  mucosum,  mucous  net;  a 
name  for  the  deep  and  soft  part 
of  the  cuticle. 

Rigor  mortis,  rigidity  of  death. 

Rima  glottidis,  fissure  of  the  glot- 
tis. 

Sacrum,  sacred;  the  bone  which 
forms  the  part  of  the  vertebral 


column  succeeding  the  lumbar 
vertebrae,  and  articulating  with 
the  pelvic  bones ;  an  object  of 
superstitious  regard,  probably 
on  account  of  its  triangular 
shape. 

Sarcolemma,  <rcip%  flesh,  and  Xe>- 
/*«  a  husk ;  the  membrane  which 
surrounds  the  contractile  sub- 
stance of  a  striped  muscular 
fibre. 

Scala,  a  ladder  or  staircase.  The 
scala  tympani  and  scala  vesti- 
buli  are  the  two  passages  filled 
W7ith  perilymph  in  the  cochlea. 

Scaphoid,  <rKa<pri  a  hollow  vessel 
or  boat,  and  el<5os  form;  the 
name  of  one  of  the  carpal  and 
one  of  the  tarsal  bones. 

Scapula,  the  shoulder-blade. 

Sclerotic,  o-/cX?jpos  hard;  the  tough 
coat  of  the  eye  ball,  which,  with 
the  cornea,  forms  its  outer  wall. 

Sebaceous,  sebum  tallow ;  seba- 
ceous glands,  those  which  se- 
crete the  oil  of  the  skin. 

Semilunar,  semilunaris  half -moon 
shaped;  the  name  of  a  carpal 
bone. 

Sensorium,  sentio  I  perceive  by 
the  senses ;  the  nervous  centre 
which  must  be  reached  by  sen- 
sory impressions  before  they  can 
be  perceived. 

Sequestrum,  sequestra  I  set  aside ; 
a  dead  portion  of  bone  separated 
or  destined  to  separate  from  the 
living  parts. 

Serum,  whey;  the  fluid  part  of 
the  blood,  separated  from  the 
fibrin  and  corpuscles ;  serous 
membrane,  a  membrane  form- 
ing a  shut  sac,  and  secreting 
serum  or  fluid  sufficient  to  lub- 
ricate its  opposed  surfaces. 

Sesamoid,  (niaa/mov  a  kind  of  seed, 
and  et<5os  form;  seed-like.  Sesa- 
moid bones  are  those  like  seeds 
in  tendons. 

Sigmoid,  2  and  ttoos  form;  shaped 
like  an  S,  as  the  sigmoid  flexure 
of  the  intestine. 

Sinus,  a  hollow.  Osseous  sinuses 
are  hollows  in  bones,  filled  with 
air,  as  the  frontal  sinus.  The 


316 


GLOSSARY. 


venous  sinuses  in  the  interior  of 
the  cranial  cavity  are  hollows  in 
the  dura  mater  which  perform 
the  function  of  veins. 

Sinus  pocularis,  cup-like  sinus ; 
a  minute  hollow  in  the  prostatic 
portion  of  the  urethra,  repre- 
senting the  uterus  in  the  female. 

Smegma,  soap ;  the  white  soapy 
substance  frequently  adherent 
to  the  skins  of  new-born  infants. 

Solar,  sol  the  sun.  The  solar 
plexus  is  the  large  plexus  of 
sympathetic  nerves  in  the  upper 
part  of  the  abdomen. 

Soleus,  solea  a  sole;  the  name  of 
a  muscle  of  the  calf  of  the  leg, 
shaped  much  like  a  sole. 

Somnambulism,  somnus  sleep,  and 
amhilo  I  walk;  walking  in  sleep. 

Spectrum,  an  appearance ;  in 
physics,  the  prismatic  colours 
obtained  by  analysis  of  the  rays 
of  any  luminous  body ;  in  phy- 
siology, the  image  which  con- 
tinues to  be  seen  after  gazing  at 
any  bright  object.  ui 

Spermatozoon,  oW/o^rf  a  seed,  and 
£ooV  a  living  thing;  the  essen- 
tial male  element  of  reproduc- 
tion. 

Sphacelus,  a-tyaKeXos  gangrene. 

Sphenoid,  o-cprju  a  wedge,  and 
6i«5o5  form ,  the  name  of  the 
central  bone  in  the  base  of  the 
skull. 

Sphincter,  a-<j>tyKvnp  a  tight  band; 
a  circular  muscle  which  keeps 
an  orifice  habitually  shut. 

Sphygmograph,  <7</>uy/xos  the 
pulse,  and  ypafpio  I  write ;  an 
instrument  with  a  lever  which 
rises  and  falls  with  the  pulse, 
and  has  a  pen  attached  to  it,  by 
means  of  which  it  makes  a 
tracing  on  a  card  moved  by 
clock-work. 

Spore,  o-Tropa  a  seed;  a  vegetable 
germ  which  develops  without 
impregnation. 

Scruamous,  squama  a  scale ;  con- 
sisting of  scales,  as  ^.qtiamous 
epithelium;  shaped  like  a  scale, 
as  the  squamous  part  of  the 
temporal  bone. 


Stapes,  a  stirrup ;"  the  name  of  a 
stirrup-shaped  ossicle  in  the 
tympanum  of  the  ear. 

Stercorin,  stercus  dung;  a  crys- 
tcilline  substance  obtained  from 
foeces. 

Stereoscope,  o-repeos  solid,  and 
a-KoTreca  I  behold;  an  instrument 
by  means  of  which  two  views, 
such  as  might  be  presented  by 
one  object  to  the  two  eyes,  being 
exhibited  one  to  each  eye,  a 
single  picture  is  seen  with  the 
solidity  and  perspective  of 
reality. 

Sternum,  o-Te'pi/oy  the  breast ;  the 
breast-bone. 

Stigma,  vTiyfjiri  a  puncture ;  an 
opening  leading  into  a  respira- 
tory trachea  in  an  insect;  the 
part  of  the  pistil  of  a  flower  to 
which  the  pollen  is  applied,  and 
which  leads  into  the  ovary. 

Stroma,  o-Tpw/xa  a  thing  spread 
out  for  lying  on ;  the  ground- 
work of  a  texture,  in  which 
other  parts  are  imbedded;  the 
matrix  of  a  tissue. 

Styloid,  O-TU\OS  a  pointed  instru- 
ment for  writing,  and  el8o<s 
form ;  the  name  of  certain  pro- 
cesses, as  the  styloid  process  of 
the  ulna  and  of  the  temporal 
bone. 

Succus  intestinalis,  iutestin.il 
juice;  the  secretion  of  the 
glands  of  the  mucous  membrane 
of  the  small  intestine. 

Sudoriparous,  suclor  sweat,  and 
pario  I  bring  forth  ;  the  sudori- 
parous glands  secrete  the  per- 
spiration. 

Sutura,  sutura  a  seam.  While  in 
surgery  it  is  applied  to  any  seam 
for  closing  a  wound,  is  in  ana- 
tomy applied  to  an  articulation 
in  which  iwo  edges  of  bone  are 
immovably  united,  with  only 
periosteum  between  them. 

Sympathetic,  <r(>v  together  with, 
and  -rraOos  suffering;  the  name 
given  to  the  ganglionic  system 
of  nerves,  on  account  of  its  con- 
nection with  the  cerebro-spinal 
system. 


GLOSSARY. 


317 


Symphysis,  o-vv  together,  and 
$uovs  growth ;  a  name  applied 
to  certain  instances  of  incom- 
plete articulation  dissimilar  in 
their  nature,  as  the  symphysis 
pubis  and  symphysis  of  the 
lower  jaw. 

Synchronous,  cvv  together,  and 
Xpo'i/ov  time;  occurring  at  the 
same  time. 

Syncope,  o-vyKOTrtj  a  swoon ;  un- 
consciousness, from  failure  of 
the  heart's  action. 

Synpstosis,  avv  together,  and  60-- 
-reov  bone  ;  used  with  reference 
to  the  bones  of  the  skull;  means 
the  premature  obliteration  of 
sutures. 

Synovia,  a-vv  together,  and  uwv 
an  egg;  secretion  like  white  of 
egg;  the  fluid  lubricating  the 
interior  of  a  joint. 

Syntonin,  adv  together,  andreii/co 
I  stretch  ;  a  peculiar  variety  of 
fibrin  obtained  from  muscular 
fibre. 

Tsenia,  a  ribbon ;  tcenia  hippo- 
campi and  tcenia  semicircularis, 
parts  in  the  brain. 

Tapetum,  a  carpet ;  the  shining 
layer  existing  in  the  choroid 
coat  of  the  eye  in  many  ani- 
mals. 

Tarsus,  T-apo-J?  the  flat  of  the 
foot ;  the  seven  bones  which 
form  the  instep  and  heel. 

Teleology,  -re'Xos  an  end  accom- 
plished, and  \6yo<5  discourse; 
the  study  of  function. 

Temporal,  tern-pus  time;  whence 
fampora  the  temples  or  sides  of 
the  head  where  the  ravages  of 
time  are  liable  to  be  shown  by 
the  whitening  of  the  hair;  the 
name  of  the  bone  at  the  side 
and  base  of  the  skull,  in  which 
the  ear  is  situated. 

Tendon,  tendo  a  noun  from  tendo 
I  stretch  ;  a  variety  of  white 
fibrous  tissue  through  the  me- 
dium of  which  muscle  is  at- 
tached. 

Tentorium  a  tent ;  tentorium  cere- 
he  process  of  dura  mater 


which  separates  the  cerebellum 
from  the  cerebral  hemispheres. 

Testis  a  witness;  the  gland  secret- 
ing the  spermatozoa. 

Tetanus,  TCTUVOS  tension  (reivw 
I  stretch) ;  the  spasmodic  or  in- 
voluntary continued  active  con- 
traction of  muscular  fibre. 

Thalanms,  VaXa/mos  a  chamber ; 
optic  thalamus,  the  name  of  a 
portion  of  the  brain  from  which 
the  fibres  of  the  optic  tract 
partly  arise. 

Thaumotrope,  Gav/ma  a  wonder, 
and  rp&iru*  I  turn;  an  instru- 
ment, in  which  figures  in  series 
of  different  positions  are  painted 
near  the  circumference  of  a 
disc,  and  the  reflections  of 
these,  being  looked  at  in  a 
mirror  through  openings  in  a 
card  revolving  with  them,  are 
seen  in  the  form  of  figures,  each 
of  which  performs  the  movement 
represented  in  stages  on  the 
disc.  This  instrument  is  also 
called  a  stroboscope  (<iTpo/3ea>  I 
whirl). 

Theca  a  sheath ;  a  synovial  sheath 
of  a  tendon. 

Thorax,  6wpa%  a  breast-plate;  the 
cavity  which  contains  the  heart 
and  lungs ;  or,  when  the  skeleton 
is  spoken  of,  the  ribs,  dorsal  ver- 
tebras, costal  cartilages,  and 
sternum. 

Thymus,  tiv/uLos  heart  or  soul ;  the 
thymus  gland,  a  ductless  gland 
in  the  upper  part  of  the  chest, 
in  early  life. 

Thyroid,  0upeJs  a  shield,  and  elSos 
form;  the  thyroid  cartilage,  the 
largest  cartilage  of  the  larynx  ; 
the  thyroid  body,  a  ductless 
gland  on  the  front  and  sides  of 
the  upper  part  of  the  trachea. 

Tibia  a  flute ;  the  large  inner  bone 
of  the  leg,  the  shin-bone. 

Tonicity,  TOVO?  tightening;  mus- 
cular contraction,  of  a  slight  de- 
gree, persistently  continuous. 

Tonsil,  tonsilla  a  structure  on  each 
side  of  the  fauces ;  also  called 
amygdala,  from  being  the  size 
of  an  almond. 


318 


GLOSSARY. 


Trabecula,  a  little  rafter. 

Trachea,  Tpa^us  rough;  the  wind- 
pipe. See  the  word  artery. 

Tragus,  Tjmyos  a  goat ;  the  emi- 
nence in  front  of  the  opening  of 
the  ear ;  sometimes  hairy,  like 
a  goat's  beard. 

Trapezium,  a  geometrical  figure; 
the  name  of  the  carpal  bone 
which  supports  the  thumb. 

Trapezoid,  a  geometrical  figure; 
the  name  of  one  of  the  carpal 
bones. 

Tricuspid,  trta  three,  and  cuspijs 
a  pointed  extremity;  the  tri- 
cnspid  valve  of  the  heart,  con- 
sisting of  three  cusps. 

Trigone,  Tpia  three,  and  ywvia  an 
angle;  the  part  of  the  bladder 
between  the  openings  of  the 
ureters  and  arethra. 

Trochlea,  a  pulley. 

Tuberosity,  tuber  a  lump ;  a  thick 
prominence  of  a  bone. 

Turbinated,  turbo  a  turning  round ; 
the  turbinated  bones,  all  more 
or  less  curved. 

Tympanum,  a  timbrel  or  drum; 
the  cavity  termed  the  drum  of 
the  ear. 

Ulna,  wXeV);  the  elbow;  the  inner 

bone  of  the  fore-arm. 
Umbilicus,  the  navel. 
Unciform,  uncus  a  hook ;  the  name 

of  one  of  the  carpal  bones. 
Urachus,  ovpov  urine,  and  e'x&o  I 

hold;    the  constricted   part   of 

the  allautois,  which  remains  as  a 

cord  ascending  from  the  bladder. 
Ureter,  ovpirn}p  the  duct  from  the 

kidney  to  the  urinary  bladder. 
Urethra,    ovpntipa    the    excretory 

duct  of  the  bladder. 
Uvula,  a  little  grape ;  the  pendant 

body  at  the  back  of  the  soft 

palate. 

Vallecula,  a  little  valley ;  the  hol- 
low in  the  middle  of  the  under 
surface  of  the  cerebellum. 


Valvulsa  conniventes,  valves  ap- 
proximating one  to  another ;  the 
folds  of  mucous  membrane 
which  project  into  the  small  in- 
testine. 

Velum  interpositum,  interposed 
curtain ;  a  fold  of  pia  mater  pro- 
longed beneath  the  fornix,  and 
supporting  choroid  plexuses. 

Velum  palatse,  the  soft  palate. 

Vense  comites,  companion  veins  ; 
two  or  more  veins  coursing  in 
company  with  an  artery. 

Ventricle,  ventrlculm  (venter  a 
belly)  a  little  cavity.  The  ven- 
tricles of  the  heart  are  the 
cavities,  the  walls  of  which  pro- 
pel the  blood. 

Ventriloquist,  venter  the  belly, 
and  loquor  I  speak;  one  who 
has  the  art  of  managing  his 
voice  so  as  to  make  it  appear 
that  the  sounds  emanate  from 
a  different  direction. 

Vermicular,  vermis  a  worm ;  ver- 
micular movement  in  waves, 
such  as  are  seen  in  a  worm. 

Vertebra,  verto  I  turn ;  a  bone  of 
the  spinal  column. 

Vestibule,  vestibulum  an  entrance. 
The  vestibule  of  the  ear  is  the 
part  of  the  labyrinth  from  which 
the  semi-circular  canals  come 
off. 

Vitelline,  vitellus  the  yolk  of  an 
egg;  belonging  to  the  yolk,  as 
the  vitelliue  membrane. 

Vitreous,  v'dreus  of  glass  ;  the  vit- 
reous humor  is  named  from  its 
glassy  appearance. 

Zona  pellucida,  the  pellucid 
zone  surrounding  the  yelk  of 
the  unimpregnated  ovum. 

Zonule  of  Zinn,  the  plicated  zone 
formed  by  the  suspensory  liga- 
ment of  the  lens  of  the  eye. 

Zygoma,  tuyov  a  yoke;  the  arch 
formed  by  the  malar  bone  and 
zygomatic  process  of  the  tem- 
poral bone. 


INDEX. 


ABOMASUM,  96. 

Absorbent  System,  151. 

Absorption,  150. 

Accommodation  to  Distances,  242. 

Acetabulum,  31. 

Acromion,  28. 

Agminated  Glands,  104 

Air  Cells,  136. 

Albinos,  231. 

Albuminoids,  78. 

Albuminoid  Textures,  22. 

Alimentation,  76. 

Allantois,  28-4. 

Alternate  Generation  of  Aurelia  Aurita, 
275. 

Alternate  Generation  of  Hydroid   Zoo- 
phyte, 274. 

Ammonia,  12, 

Amoeba,  17. 

Amyloids,  166. 

Analogy  of  Nerve  Terminations,  238. 

Anatomy,  9. 

Animals,  Functions  of,  13. 

Aunulus  Ovnlis,  117. 

Anterior  Pyramids,  193. 

Aorta,  117. 

Aporieurosis,  22. 

,,  Human,  23. 

Apparatus  Lachrymal,  252. 

Apparitions,  213. 

Aqueous  Humour,  235. 

Arachnoid  Membrane,  185. 

Areolar  Tissue,  20. 

Arterial  Blood,  112,  161. 

Arterial  System,  123. 

Arterial  Valves,  119. 

Arteries,  122. 

Arteries,  Elasticity  of,  124. 

Arteries,  Hypogastric,  295,  297. 

Arteries,  Pulse  of,  125. 

Artery,  Pulmonary,  117. 

Arthrodia,  47. 

Articular  Cartilage,  41. 

Arytenoid  Cartilages,  268. 

Ascending  Colon,  94, 

Asphyxia,  112,  145,  146. 

Astragalus,  33, 


Atlas  and  Axis,  34. 
Atmosphere,  143. 
Auditory  Nerve,  205. 
Auricles,  114. 
Axis  of  the  Eye,  238. 

BACILLARY  Layer,  233. 
Basilar  Membrane,  263. 
Bees,  Reproduction  of,  275. 
Bicuspid  Valves,  120. 
Bile,  100,  102,  165. 
Biology,  10. 
Bladder,  282. 
Blood,  105. 

„      Arterial  and  Venous,  112,  161. 

„      Capillaries  of,  114. 

„      Chemical  Composition  of,  109. 

,,      Coagulation  of,  105. 

,,      Circulation  of,  114. 

Composition  of,  111. 

Corpuscles  (Human),  106. 

Corpuscles  of  Frog,  107. 

Flow  of,  129. 

Plasma  or  Liquor  Sanguinis,  106. 

Time  of  Circulation,  130. 

Vessels  of  Mucous  Membrane  of 

Stomach,  155. 
Body,  Centre  of  Gravity,  49. 
Bone,  42. 

Bone-Corpuscles,  42. 
Bones  of  the  Hand,  30. 
Brachial  Plexus,  189. 
Brain,  Diagram  of,  197. 
Branchial  Processes,  289,  294. 
Bronchial  Tubes,  135. 
Bronchitis,  135,  140. 
Bronchus,  133,  134. 
Buccal  Cavity  and  Pharynx,  89. 
Buccal  Glands,  97. 

C^CUM,  93. 

Calcaneo- cuboid  or  Plantar  Ligaments, 

34. 

Calcaneum,  33. 
Caneliculi,  42. 
Cancellated  Tissue,  42,  45. 
Capillaries  Magnified,  127 


320 


INDEX. 


Capillary  Network,  It. 

Carbohydrates,  79,  80. 

Carbon,  12. 

Carbonaceous  Foods,  78. 

Carbonic  Acid,  11,  70,  143. 

Carpal  Bones,  30. 

Cartilage,  40. 

Cartilage  and  Muscles  of  External  Ear, 
254. 

Cartilages  of  the  Larynx,  267. 

Cast  of  Infundibula,  136 

Caudal  Vertebra,  31. 

Cerebellum,  194. 

Cerebral  Hemispheres,  195,  208. 

Cerebration,  Unconscious,  212. 

Cerebro-spinal  System,  176 

Cerebro-spinal  Axis,  177,  184. 

Cerebro-spinal  Nervous  System,  177. 

Cerebrum,  195. 

Cervical  Plexus,  189. 

Check  Ligaments,  35. 

Chemical  Composition  of  Blood,  109. 

Chlorophyll,  80. 

Cholesterin,  165. 

Cholic  Acid,  165. 

Chondrin,  40. 

Chorda  Dorsalis,  284,  287. 

Chorda  Tympani,  64,  113,   205. 

Chordae  Tendineoe,  120. 

Chorion,  283. 

Choroid,  236. 

Choroid  Plexus,  200. 

Chyle,  154. 

Chyme,  93. 

Cicatricula,  285. 

Ciliary  Processes,  231. 

Circulation  of  Blood,  114. 

Clavicle,  28. 

Cleavage  of  Cicatricula  of  Hen's  Egg,  286. 

Cleavage  of  the  Yelk,  283. 

Closed  Follicles,  104. 

Coagulation  of  Blood,  105 

Coccyx>  31. 

Cochlea,  262. 

Cochlea  of  New-born  Pig,  2o3. 

Cod,  The  Brain  ofv  199. 

Colloids,  156. 

Colour-blindness,  248. 

Columnar,  Epithelial  Cells  of  Intestine, 
155. 

Combustibility,  12. 

Compact  or  Bony  Tissue,  42. 

Compact  Osseous  Tissue,  Transverse  Sec- 
tion, 43. 

Complete  Joints,  47. 

Common  Sensation,  218. 

Composition  of  Blood,  111. 

Compound  Sacculated  Glands,  63. 

Connective-tissue-corpuscles,  20. 

Contractility,  16. 

Contraction  of  Auricles,  119. 

Convolutions  of  Brain,  195. 

Coracoid  Process,  29. 

Cornea,  228. 


Corpora  Quadrigemina,  207. 

Corpora  Albicantia,  198. 

Corpora  Striata,  198,  207. 

Corpuscles,  Nucleated,  17,  18,  176. 

Corpus  Luteum,  281. 

Connective  Tissue  from  the  Orbit  of  the 

Ox,  21. 

Costal  Cartilage,  41. 
Coughing,  206. 
Course  of  the  Ingesta,  88. 
Cranial  Nerves,  204. 
Cranium,  35. 
Cricoid  Cartilage,  267. 
Crura  Cerebelli,  193. 
Crura  Cerebri,  207. 
Cruorin,  113. 
Crusta  Petrosa,  85. 
Crystalline  Lens,  233,  239,  240. 
Crystalloids,  156. 
Cuboid,  33. 
Cuneiform  Bones,  30. 
Cuticle,  66. 
Cutis  Vera,  67. 

DALTONISM,  or  Colour-blindness.  249 
Death,  300. 

Decidua,  Diagram  of,  283. 
Decidua  Reflexa,  283. 
,,        Serotina,  284. 

Vera,  283. 

Deep  Surface  of  Soleus  Muscle,  59. 
Deglutition,  90, 
Dentine,  84. 
Derma  (Cutis),  67. 
Descending  Colon,  94. 
Development,  10. 

, ,  and  Reproduction,  273. 

„  of  Bone,  44. 

,,  of  Brain  and  Spinal  Cord, 

184,  201,  286. 
,,  of  Heart,  293. 

„  tf  Kidneys.  170. 

,,  ->f  Limbs,  289. 

„  of  the  Eye,  235. 

,,  of  the  Face  and  Palate  290. 

„  of  the  Teeth,  85. 

Diagram  of  Brain,  197. 
„        of  Decidua,  283. 
,,        of  Insect's  Eye,  237. 

of  the  Right  Ear,  256. 
„        of    the    Seminifei-ous    Tubules 

and  Ducts,-  281 
„        to  Illustrate  the  Course  of  Two 

Cones  of  Light,  240. 
Diaphragm  (MMriff),  138. 
Diaphragmatic  Breathing,  138. 
Dichrotic  Pulse,  127. 
.  Digestion,  Definition  of,  82. 
j  Digestive  Fluids,  96. 
Tubes,  92. 
Disinfectants,  147. 
Dorsal  Plate,  288. 
Double-headed  Embryos,  276. 
PI  earns,  213 


INDEX. 


321 


Ductless  Glands,  Liver,  and  Kidneys,  159. 
Duodenum,  93,  101,  102. 

EAR  (Human),  254. 
Effects  of  Distance  on  the  Eyes,  246. 
Embryo  Chick,  287. 
„        Lamb,  290. 
Embryos,  Double-headed,  276. 
Enamel,  85. 
Encephalon,  Structure  of,  193. 

„  Functions  of,  206. 

Endosmosis,  156. 
Endothelium,  127. 
Epidermal  Appendages,  71. 
Epiglottis,  90. 
Epiphyses,  46. 
Epithelium,  61. 

„          Ciliated,  62,  225. 

,,  Columnar,  62. 

,,  Glandular,  62. 

,,  Squamous,  61. 

Varieties  of,  61. 

Btbmoidal  Turbinated  Bones,  222. 
Ethmoid,  35. 
Eustachian  Tube,  257. 
Excretin,  103. 
Exosmosis,  156. 
Expiration,  143. 

External  Auditory  Meatus,  36,  255, 
,,        Cuneiform  Bones,  32. 
„        Ear,  Cartilage  and  Muscles  of, 

254. 

„        Ear,  Description  of,  255. 
Eye,  Focus  of,  242. 
Eyeball,  228. 
Eyelids,  251. 
Eyes,  Effects  of  Distance  on,  246. 

PACE  Bones,  36. 

Faeces,  103. 

Facial  Nerve,205. 

Foatal  Circulation,  296. 

Foetus,  284. 

Fascia,  22. 

Fauces,  88. 

Febrile  Affections,  149. 

Femur,  or  Thigh  Bone,  32. 

Fenestra  Rotunda,  258. 

Fibre  Cells  of  Unstriped  Fibre,  55. 

,,     Striped  Muscular,  54. 
Fibrillse,  54. 
Fibrin,  105,  110. 
Fibrinogen,  110. 
Fibrinoplastin,  110. 
Fibro-Cartilages,  42. 
Fish,  Heart  and  Great  Vessels  of,  115. 
Flavour,  227. 
Flexor  Muscles,  129. 
Focus  of  the  Eye,  242. 
Foot,  33. 

Foramen  Magnum,  35. 
Fossa,  117. 

Frog,  Heart  and  Great  Vessels  of,  115 
Frog's  Luug,  134. 

14 


Front  View  of  the  Bones  of  the  Hand,  30. 
Functions  of  the  Eucephalou,  20t>. 

GALL  Bladder,  165 

Galvanometer,  57w 
Ganglia,  122,  176. 
Ganglion,  176. 
Gastric  Follicles,  99. 

„       Juice.  98. 
Gelatinoids,  78. 
Germinal  Membrane,  284. 
Gestation,  Period  of  in  Human  Species, 

297. 

Giddiness  and  Nausea,  221. 
Glands  (Brilnner's),  100. 

,,      Compound  Sacculated,  63. 
Gland  Lachrymal,  251. 
Glands,  Peptic,  99. 
Gland,  Submaxillary,  97. 
Glossopharyngeal  Nerve,  205 
Glotti.^  89. 
Glutei  .Muscles,  38, 
Glycogen,  166. 
Graafian  Vesicles,  279. 
Grey  Matter  of  the  Convolutions,  196. 
Growth  after  Birth,  299. 
Growth  of  Head,  300. 
Gums,  88. 

HAEMOGLOBIN,  111. 

Hair,  72,  73. 

„    Development  of,  75. 
Hand,  Bones  of,  30. 

„      Front  View  of  the  Bones  of,  3/» 

,,      Integument  of,  65. 
Haversian  Canals,  42. 
Head,  Growth  of,  300. 
Hearing,  253. 
Heart,  116,  216. 

,,      Anatomically  Considered,  117 

,,      Action  of,  118,  122., 

,,      Action,  Regularity  of,  125. 

„      Auriculo- Ventricular  Valves  of, 
120. 

„      Beating  of,  121 

„      Left  side  of,  120. 

,,      Pulsations  of,  121. 

„       Right  Side  of,  118. 

,,      and  Great  Vessels  of  Fish,  115. 

,,      and  Great  Vessels  of  Frog,  115. 
Hemispheres,  Cerebral,  195. 
Hemisphere- vesicles,  200 
Hen's  Eggs,  286. 
Hepatic  Lobule,  163. 

,,       Structure,  164. 
Hiccough,  207. 
Hilus  of  Kidney,  168. 
Histology,  11. 
Human  Aponeurosis,  23. 

„       Eye,  229. 

„      Kidneys,  170. 

, ,      Heart  and  Vessels,  116. 

„       Ovum,  277. 

„      Skeleton,  Peculiarities  of,  37. 


322 


INDEX. 


Human  Windpipe  and  Lungs,  135. 
Humerus,  or  Arm  Bone,  28. 
Hyaline  Cartilage,  40. 
Hyaloid  Membrane,  233. 
Hydrogen,  12. 
Hyoid  Bone,  89. 
Hypoaria,  20. 
Hypoglossal  Nerves,  205. 

ILEO-COLIO  VALVE,  94. 

Ileum,  93. 

Ilium,  31. 

Incisor  Tooth,  83. 

Incomplete  Joints,  46. 

Incus,  258. 

Inferior  Calcaneo-scaphoid  Ligament,  34. 

„        Maxilla,  36. 

„        Turbinated  Bones,  37. 
Infundibulum,  136,  198. 
Ingesta,  Course  of,  88. 
Insect's  Eye,  Diagram  of,  237. 
Inspiration,  Acts  of,  141. 
Integument,  The,  65. 

,,  of  Hand,  65. 

Intercostal  Fibres,  188. 
Internal  Ear,  260. 
Internal  Temperature,  148. 
Intestine,  93. 
Iris,  230. 
Irritability,  16. 
Ischium,  31. 

JEJUNUM,  93. 

Joints  or  Articulations,  46. 
Joints,  Incomplete,  46. 
Jugal  or  Malar  Bones,  37. 
Juice,  Gastric,  98. 

KANGAROO,  Kidneys  of,  169. 
Kidneys,  168. 

Cortical  Portion  of,  169. 

Human,  170. 

Medullary,  169. 

of  Kangaroo,  169. 

of  Seal,  170. 

Primordial,  168. 

Texture  of,  171. 
Kreatin,  56. 

LACHRYMAL  GLAND,  251. 
,,  Apparatus,  251. 

Bones,  37. 
Lacteals,  153. 
Lacunae,  42. 

Lacuuae  and  Canaliculi,  43. 
Lamb,  Embryo  Brain  of,  202. 
Larynx,  The,  270,  134. 

„        Cartilages  of,  267. 

,,        Mesial  Section  of,  268. 

„        Muscles  of  269. 
Lateral  Ventricles,  203. 
Laxator  Tympani  Muscle,  259. 
Leaping,  50, 


Lenticular  Glands,  104. 
Levator  Palpebrse  Muscle,  252. 

„        Palati  Muscle,  257. 
Leucocytes,  108. 
Levers,  Three  Orders  of,  51,  52. 
Lieberktthn's  Follicles,  101. 
Ligament,  23. 

„          Orbicular,  30. 
Ligamentum  Denticulatum,  185. 

„  Nuchse  of  the  Horse,  30. 

Liver,  102,  162,  167. 
Lobules  of  Human  Lung,  137. 
Lobule  of  Liver  of  Oyster,  64. 
Lobule    of   Parotid   Gland  of  Embryo 

Lamb,  64. 

Localization  of  Impressions,  20. 

Lumbar  and  Sacral  Plexus,  189. 

,,        Intervertebral  Disc,  47. 

Vertebra,  31. 
Lymph,  152. 

„      Corpuscles,  153. 
Lymphatic  Vessels,  150. 
„  Glands,  109,  152 

MALAR,  or  Jugal  Bones,  37. 

Malleus,  258. 

Malpighian  Corpuscles,  161. 

„  „  Relations  of,  172. 

„  „  Structure  of,  173. 

Mammae,  298. 

Mammalian  Brain  in  a  Festal  Stage,  201. 
Mastication,  90. 
Mastoid,  36. 
Mastoid  Cells,  258. 
Matrix  of  Nail,  71. 
Maxillary  Lobe,  290. 
Medulla,  73. 

„        Oblongata,  206,  193. 
Medullary  Kidneys,  169. 
Membrana  Nictitans,  253. 

„          Tympani,  254,  256,  205 
Membrane  Arachnoid,  185. 

,,         Basilar,  263. 
Membranous  Labyrinth,  261. 
„  Vestibule,  261. 

Mesenteric  Glands,  153. 
Mesial  Section  of  Brain,  195. 

„  ,,       of  Larynx,  268. 

Metacarpal  Bones,  30. 
Middle  Ear,  257. 
Milk,  Constituents  of,  298. 
Modiolus,  262. 
Molar  Tooth,  84. 
Movable  Articulations,  46. 
Mucous  Membrane,  60,  98. 
Multipolar  Nerve-corpuscle,  183. 
Muscles,  Intercostal,  140. 

„        of  the  Eyeball,  243. 

,,        of  the  Larynx,  269. 
Muscular  Fibre,  53. 

„  „      Striped. 

„        Tissue,  Unstriped,  55. 
Musculi  Papillares,  120. 
Myosin,  55 


INDEX. 


323 


NAIL  and  its  Matrix,  71. 
Nasal  Duct,  222,  253. 
Nasal  Fossse,  221. 
Nausea  and  Giddiness,  221. 
Necrosis,  301. 
Nerve-corpuscles,  182. 

„  Multipolar,  183. 

Nerve-fibres,  181. 
Nerve,  Electric  Properties  of,  180. 

,,        Terminations,  176. 
Nervous  Action,  178. 
Nervous  System,  176. 

„  „        Functions  of,  179,  193. 

Tissues,  181. 
Nitrogen,  77,  143. 
Nitrogenous  Foods,  78. 

,,          Substances,  11. 
Non-nitrogenous  Substances,  11. 
Nucleated    Corpuscles.   Multiplication 

of,  18. 

Numbness,  221. 
Nutrient  Artery,  44. 
Nutrition,  13. 

OCULAR  SPECTRA,  249. 
Ocelli  of  Star-fish,  237. 
Occipital  Bone,  35. 
Odontoid  Process,  35. 


Olfactory  Bulbs,  198. 
„         Cells,  224. 
,.        Nerves,  223,  204. 
Olivary  Bodies,  193. 
Omasum,  96. 
Omphalo-mesentric  Veins,  292. 

„  „         System,  292. 

Optic  Lobes,  199. 

„    Nerves,  198,  200,  204,  236. 

„    Pore,  232. 

„    Thalami,  198. 
Ora  Serrata,  232. 

Orbicularis  Palpebrarum  Muscle,  252. 
Orbicular  Ligament,  30. 
Organ  of  Corti,  264. 
Organic  Matter,  11. 
Organisms,  11. 
Organic  World,  12. 
Os  Magnum,  30. 
Osmosis,  153. 
Os  Pubis,  31. 
Osseous  Labyrinth,  260. 
Osteoblastic  Corpuscles,  45. 
Ovaries,  The,  279. 
Ovary  and  Oviduct  of  Sunfish,  280. 
Ovum,  273. 
Oxidation,  12. 
Oxygen,  12,  111,  143. 

„        Absorption  of,  149. 
„        Combination  of,  112. 
,,        in  Respiration,  112,  144. 
Oyster,  Lobule  of  Liver,  64. 

PACINIAN  BODIES,  68. 


Pacinian  Corpuscles,  and  Sweat  Glands, 
69. 

Palatals,  37. 

Pancreas,  100,  102. 

Pancreatic  Juice,  100. 

Papillae  of  the  Tongue,  224. 

Paraglobulin,  110. 

Parietals,  35. 

Parotid  Gland,  96. 

Patella  or  Knee-Cap,  32. 

Paunch,  96. 

Peculiarities  of  Human  Skeleton,  37. 

Pelvic  or  Innominate  Bones,  31. 

Pelvis,  31,  170. 

Pepsin,  98. 

Peptones,  98. 

Periosteum,  43. 

Petrous  Part,  36. 

Peyers  Patches,  104. 

Pigment  Corpuscles  of  the  Choroid  Coat, 
230. 

Phalanges,' 30, 32. 

Pharynx,  88. 

„        and  Buccal  Cavity,  89. 

Phosphenes,  250. 

Phrenology,  210. 

Physiology,  9. 

Pia  mater,  185. 

Pineal  Body,  198. 

Pisiform,  30. 

Placenta,  284. 

Plan  of  the  Branchial  Arches  in  Mam- 
mals, 293. 

Pleura,  137. 

Plexus,  178. 

Plica  Semilunaris,  253. 

Pneumogastric  Nerve,  205. 

Pons  Varolii,  193. 

Portal  System,  131. 
„      Vein,  132. 

Posterior  Pyramids,  193. 

„        Half  of  Eyeball,  233. 

„        Sacro-Iliac  Ligaments,  32. 

Primitive  Groove  of  Babbit,  285,  286. 

Primordial  Vertebrae,  288. 
„  Aorta,  294. 

„          Kidneys,  168. 

Process  Coronoid,  29. 

Protagon,  182. 

Proteids,  16. 

Protoplasm,  16. 

Pterygoid  Processes,  37. 

Ptyalin,  97. 

Pulmonary  Artery,  117,  295. 

Pulse  Dichrotic,  127. 

Pulses,  Sphygmographic  tracings  of,  126. 

Pulseless  Trance,  302. 

Pus,  108. 

Pylorus,  92. 

Pyramids  of  Malpighi,  170. 

RADIUS  and  Ulna,  29, 
Receptaculum  chyli,  150. 
Rectum,  94,  95, 


INDEX. 


Reproduction,  15. 

,,  and  Development,  273. 

Respiration  and  Temperature,  133. 

„          Obstruction  of,  145. 
Respiratory  Rhythm,  138. 
Rete  Mucosum,  66. 
Reticular  Cartilage,  41. 
Reticulum,  96. 
Retina,  232,  234,  236. 

„       Impressions  on,  247. 
Retinal  Vessels,  251. 
Rima  Glottidis,  268. 
Ruminant  Stomach,  96. 

SACRUM,  31. 
Saliva,  179,  96. 
Salivary  Glands,  97. 
Sarcolemma,  54. 
Scalp,  Section  of,  74. 
Scaphoid,  33. 

„        Semilunar,  30. 
Scapula,  28. 
Sclerotic  Coat,  228. 
Seal,  Kidneys  of,  170. 
Sebaceous  Glands,  70. 
Secretion,  63. 
Section  of  Coroid  of  the  Ox,  230. 

,,        of  Human  Eye,  232. 

,,       of  Mammalian  Ovum,  285. 
of  Scalp,  74. 

,,       of  Foot,  34. 

„       of  Pelvis,  32. 

„       of  an  injected  Tonsil,  103. 
Secreting  Corpuscles,  64. 
Semilunar  Ganglia,  215. 
Sense  of  Temperature,  219. 
Senses,  number  of,  218. 
Sensation,  15. 
Sensorium,  209. 

,,         Commune,  210. 
Sensory  Nerves,  225. 
Serous  Membranes,  60. 
Serum,  105. 
Sesamoid  Bone,  32. 
Sexual  Reproduction  in  Vertebrata,  276. 

„      Reproduction,  273. 
Shoulder,  29. 
Sigmoid  Flexure,  94. 
Skeletal  Textures,  40 
Skeleton,  The,  26. 
Skeleton,  Mechanics  of,  48. 
Skin,  Sensibility  of,  68. 
Skull,  35,  36. 
Sleep,  212. 

Smell,  the  sense  of,  221. 
Sneezing,  207. 
Solar  Plexus,  215. 
Soleus  Muscle,  59. 
Solitary  Glands,  104. 
Somnambulism,  213. 
Species  of  Amoeba,  17, 
Spectra,  Ocular,  249. 
Speech,  271. 
Sphacelus,  301, 


Sphenoid,  35. 
Sphincter,  95. 
Spinal  Arachnoid,  185. 

,,      Accessory  Nerve,  205. 

„      Cord,  186,  187. 

„      Cord,  Experiments  on  Functions 
of,  189. 

„      Ganglia,  188. 

„      Nerves,  188. 
Spleen,  102,  161,  162. 
Squamous  Portion,  36. 
Stapes,  258. 
Stercorin,  103. 
Sternum,  27. 
Stigmata,  133. 
Stomach,  91. 

„         Ruminant,  96. 
Striped  Muscular  Fibre,  54. 
„  Tissue,  54. 

Structure  of  Encephalon,  193. 
Superior  Maxillary  bones,  36. 
Supra-renal  Capsules  and  Kidney,  160. 
Suspensory  Ligament,  234. 
Sutures  or  Immovable  Articulations,  46. 
Sympathetic  System  of  Nerves,  177,  214. 
Symphysis  Pubis,  31. 
Syncope,  146.1 
Synodontis,  83. 
Synostosis,  46. 
Synovia!  Membrane,  47,  60. 

„       Bursse,  60. 
Syntonin,  55. 

TAPETUM,  231. 

Tarsus  and  Metatarsus,  32. 

Taste,  224. 

Taste-cones  of  Sheep,  226. 

Teeth,  82. 

Teeth,  Permanent  and  Temporary,  87. 

Temperature  and  Respiration,  133. 

Temperature,  Internal,  148.J 

Tendon,  22. 

Tentorium  Cerebelli,  195. 

Tensor  Tympani  Muscle,  259. 

Textural  Elements,  11. 

„       Organs,  11. 
Texture  of  Kidneys,  171. 
Thoracic  Duct,  150. 
Thorax,  140. 

Three  Orders  of  Levers,  51,  52. 
Thymus  Gland,  159. 
Thyroid  Body,  159. 
Thyroid  Cartilage,  267. 
Thyroid  and  Thymus  Bodies,  160. 
Tibia,  46. 

„     and  Fibula,  32. 
Tissue  Areolar,  20.  j 

„      Adipose,  24. 

„     Cancellated,  42,  45 

„     Elastic,  23,  24. 

„     Striped  Muscular,  54. 

,,     Connective,  20,  21. 

„     White  Fibrous,  21,  22, 
Tongue,  225 


INDEX. 


325 


Tonicity,  57. 

Tonsils,  88,  104. 

Tonsil,  Injected  Section  of,  103. 

Tooth,  Incisor,  83. 

„      Development  of,  86. 
Touch-corpuscles,  68. 
Trachea,  132, 134. 
Transverse  Colon,  94. 
Transverse  Section  of  Cheek,  288. 
Trapezium,  30. 
Trapezoid,  30. 
Tricuspid  Valve,  120. 
Trifacial  Nerves,  204. 
Truncus  Arteriosus,  293. 
Tubuli  Uriniferi,  169. 
Tunica  Vasculosa,  230. 
Turkey,  Brain  of.  201. 
Turtle,  Brain  of,  200. 
Tympanic  Ossicles,  258. 

„  „        of  Bight  Ear,  259. 

„         Plate,  236. 
Tympanum  (Drum  of  the  Ear),  254,  257. 

UMBILICAL  ARTERIES,  295 
,,         Cord,  295. 
Vein,  295. 
„         Vesicle,  291. 
Unciform  Bones,  30. 
Under  Surface  of  the  Brain,  194. 
Unconscious  Cerebration,  212. 
Urethra,  282. 
Ureter,  168. 
Urinary  Bladder,  173. 
Urine,  174. 

„      Constituents  of,  174. 
Uterus,  Description  of,  278. 

„      and  Ovaries,  278. 

,,      of  Sheep,  279. 
Uvula,  88. 

VALVE  of  Vieussens,  198. 


Valves,  Bicuspid,  120. 

„      of  the  Arteries,  119. 
Valvulse  Conniventes,  100. 
Vascular  System,  Development  of,  292. 
Vaso-motor  nerves,  124,  215. 
Veins,  Description  of,  128, 
Velum  Palati,  88. 
Vena  Cava,  148, 

,,    Terminalis,  292. 
Verne  Comites,  128. 
Venous  Valves,  129. 
,,      Blood,  112,  161. 
„      System,  131. 
Ventilation,  147. 
Ventral  Plate,  288. 
Ventricles,  114. 

„         Contraction  of,  119. 

Lateral,  203. 
Ventriloquist,  265. 

Vermicular  or  Peristaltic  Movement,  91. 
Vermiform  Appendage,  93. 

„         Process  Inferior,  194. 
,,  „      Superior,  194. 

Vertebr89,  Arches  of,  46. 
Vertebral  or  Spinal  Column,  27,  28. 
Vertical  Transverse  Section  of  Chest  and 

Stomach,  139 
Villi,  99,  154. 
Vision,  227. 
Vitelline  Duct,  291. 
Vitreous  Humour,  233. 
Vocal  Chords,  268. 
Voice,  267. 

Voice  and  Speech,  247. 
Voluntary  Movement,  15. 
Vomiting,  207. 

WALKING  and  Running,  50. 
Wolffian  Bodies  of  Embryo  Pig,  169. 

ZYG^MATFO  PROCESS,  37. 
Zonule  of  Zinn,  234. 


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